Thanks for the comments; glad to have a critique.
Why should the majority of the population, not located anywhere near the west coast care about any of this? European influence in the Atlantic would have been by far the main concern. A Kongo colony would help defend the Atlantic. Furthermore, Zaire would surely entice more colonists than Alaska. More than Wyoming, Montana, or North Dakota for that matter as well. Lastly, a large number of natives didn't stop European colonization of South Africa or the Philippines.
I hate to say OTL, but that's pretty much the OTL decision behind it. Granted, it
was called Sward's Folly for the reasons you're listing: the majority of the population doesn't care about tundra; its benefits were strategic at that time (it allowed the US to deny it to British North America, at the very least) and as I pointed out, it removed a European nation completely from the North American game. Even if it wasn't in the Atlantic, it is one less nation to deal with.
And the Congo basin would acquire more investors and such, but I'm not sure it would attract colonists - at least not until all the final land claims were divided up in North America. (We are talking about a Congo that had a European population in the low thousands after decades of colonization). That, and my premise would be for this Congo to not include Katanga, so the best region for European habitation by climate patterns is out of consideration.
South Africa has a much more hospitable clime, and the Philippines are much more accessible, being an island chain. Most of the Congo is far from the coast and hard to access; that which is easy to access is along the Congo river littoral.
Successful German colonies are not far removed from the timeline. Bismarck had the intelligence to use an American Kongo to his advantage. Now the question is: which country would be the most unhappy about a strong American presence in the Atlantic? Great Britain, since it's their hegemony which is threatened. The next question is: what are the consequences of a timeline where Great Britain and the U.S are not on as good of terms as otl? Germany and France should have the most to gain, especially colonially.
I did posit a cooler US-British relationship being part of what leads to the entire event. Competition in Pacific islands dating back to the 1820s, the US getting the lion's share of the Oregon territory, an on-again-off-again brinksmanship and client war in Borneo with Sarawak on one side, Ambong on the other, and Brunei caught in the middle.
Germany could possibly gain more OTL (assuming it forms relatively on schedule). Italy and Austria-Hungary, even, could do better. Though, I'd imagine more
France could go either way, though I'm currently seeing them actually being rebuffed more in the Pacific.
Since American laborers are colonizing Zaire, other European powers are likely to imitate this model. Sort of like what occurred in Philippines and South Africa, but with railroads, telegraph lines, and a greater extraction of natural resources. Since American workers will build faster, they will be sent over. Even some Chinese could help, as they did in California. If relations with Great Britain aren't too bad, they might be able to strike a deal with the U.S to bring Indian laborers.
Chinese laborers would be interesting, but the territory would fall under a similar model to the Philippines OTL - acquisition, consolidation, and preparation for independence. (Eventually. and after plenty of the territory is exploited). I haven't really gamed this out much into the 20th century - too much changes in the spanners. Still, I'd expect low American population, with most of it concentrated either in mission/education work and around plantation centers.
Though, you might get a situation where the coastal region feels integrated with the US, and the inland doesn't, and the colony is split again.
Although, with worse British and American relations, the next point to consider what wars are like. Maybe the Boer war never happens. Belgium would be wise to colonize South Africa with the Dutch. The rest of Europe could sanction this, preventing Great Britain from ousting the Dutch. Now the British are likely to colonize somewhere else instead. This could result in Germany or France having slightly less territory. Ethiopia is on the table, as are current and former Spanish territory. Alternatively, the British could just simply send more people to Canada, and Oceania instead of acquiring a similar level of mercantile colonies as otl. The later choice will probably lead to the most peaceful scenario in the 20th century.
Belgium may never colonize at all - Leopold, if he lasts so long, may never have the same ambitions, or they may just be thwarted. Belgium may just remain a nation without a colony, and not deal with the investment that such an endeavor would cost.
The success of the Boers is really up in the air, as they are on valuable land that Britain wants. Although, they could potentially be bypassed if the British are in a rush to the interior to create their Cape to Cairo (assuming here, of course, that they get Egypt, which is hardly guaranteed); the Boers could simply ally with the British and possibly delay the wars some time, trading time for the British ability to lay track in their land. Probably isn't preferable without at least one war, though, to show the Boers that they could be in danger otherwise.
And, in the end, the British would need a reason to colonize, just the same as the others, regardless if the impetus is vainglorious or actually economical.
I doubt Ethiopia would be vulnerable to conquest at that point in time - again, we'd have to see how the state developed. But I'm not sure which of the changes would suggest and Ethiopia similar to OTL would be vulnerable 50 years ahead of schedule.
One thing that might happen here is the partition of Portuguese colonies by the Germans and the British, similar to one of the proposals, either 1898 or 1913. It depends of the politics at the time...
In this timeline, ocean trade is more important than otl. Germany is stronger in the Pacific in this timeline as well. Not sure how feasible more German colonization of the Pacific would be in the 20th century. Their influence in the region could be restricted to trade. China will fare even worse in this thread. The Boxer rebellion results in more European and American troops sent to China. European powers could even agree to colonize China as thoroughly as Great Britain colonized India. Japan fares worse in this thread as well. Germany, or some other power colonizes Korea. This too could involve more than one European power. Over all, there is more collaboration between European powers and America.
Don't see how sea trade is more important in general. The US Navy becomes more important than OTL; it may not have the same rot in its internal structure as it did OTL (no institutional decline, less scrapping of the majority of the fleet) and the US merchant marine becomes that much more important to maintain the connection with the overseas territories, but overall trade over the ocean should remain relatively equivalent.
I'd actually argue that there'd be very little German colonization in the Pacific; if the US is actively competing, most of it should be locked up by the time Germany even unites.
I disagree on the ability for the Europeans to actually manage a conquest of China, even then. The colonization of India was something that took centuries, and China is wealthier and more united than India was. That sort of unity in purpose seems very out of character for the squabbling powers. At the same time, I might could see spheres of influence grow and ebb.
That is a good point, though - if the US is that active to attend and be subject to a Congress of Berlin akin to OTL, they likely bother to at least cooperate more and more over time in international accords. Though, I'd point out that none of the US's tendencies toward free trade have actually shifted, and the US would likely still support the Open Door policy into China - the antithesis of the division of China into small chunk that are captive economies. That, and any attack on China that could result in any division would also greatly strengthen Russia's hand, which would be opposed by quite a few members of the Congress. Britain would worry about India, Germany and Austria-Hungary would worry about how much stronger Russia might be after grabbing large chunks of China... Overall, not a good picture for them.
Japan doesn't necessarily fare worse, though. They may not go through a Meiji revolution (or equivalent), but that doesn't mean things will be worse for them; it's more that they wouldn't be so militaristic (possibly). Korea could end up a protectorate in face of a weakening China, yes, but until China becomes weak enough on its own that this becomes possible.
Perhaps even Russia could colonize Korea and part of China in this timeline. They would be vital to harmony between European powers. In the same vein, otl Islamic states don't fare well. Russia, Great Britain, France and Germany colonize their territory more successfully than otl. The Ottoman empire either falls earlier than otl, or if not, then doesn't fall as part of a world war. One option is for Europe and America to open up the Ottoman empire in a similar manner to China and Japan. This could solely be in the interest of trade, or there could be a more thorough settlement by the west.
Really overstretching the capability of the Europeans here, I think. Russia is the one in the best position to nip even more pieces of China, compared to the OTL concessions. The Ottoman Empire and etc will probably last, especially as the Ottomans
are a European power up to this point in time. They may have been on the decline, losing territory on occasion, but in that sense it'd be like disqualifying France as a European power due to them losing Alsace-Lorraine to another.
One good thing about having a lot of strong neutral powers is that they act as buffers - once they are removed, the dividing powers start turning their eyes toward each other after a brief period of peace. Britain would hardly desire Persia or the Ottomans to weaken; it weakens their position in India and Arabia, where they've penetrate the markets quite successfully.
And why would the US care about forcing open the Ottoman Empire, which wasn't a closed society to begin with? They would never have as much success trading with the Ottomans simply due to proximity - European nations would have the advantage. There's a difference, I feel, between the US going through the motions and giving some nominal heed to a voluntary international Congress when it is in their direct interest versus the US cooperating with the Europeans in conquering a sovereign empire in a manner that will benefit those very same Europeans the most. This is especially the case, considering the US tendency to skimp on its armed forces up through WW1 or so.
In the end, nations and states (and nation-states) are always self-interested, and rarely do anything out of altruism. There is almost always an angle or a deal that is going on that influences their decisions. It isn't always the
right decision, mind, but that is the reason behind it. The relatively united intervention against the Boxer Rebellion, I feel, is more the exception that proves the rule than anything else, not to mention that the European powers at that point in time had a very weak grasp on that China - the consulates could be easily overthrown and expelled, especially when picked off one by one.
In light of all these events, then is it not likely that WWI and WWII never happen? Couldn't it also be assumed that nuclear weapons are either not proliferated, or do so much later than otl? If any country were to create them first in this timeline, I would think it's Germany, since they would lead in physics and would not have a mass exodus of physicists as Nazi's don't exist in this time line. Unfortunately, eugenics might still exist in this timeline. I'm a bit reluctant to expound upon this, but this is (awkward as it is to say), one possible solution to the encumbrances to colonizing Kongo.
A little bit deterministic (though, I know I'm guilty of that myself. Ambong existing at all, with it having formed over 50 years after the PoD?). I'd never assume that it'd be Germany, or any one state; the timeline would have to expand further. Germany, again, may never unite completely, or it may be delayed; they certainly have the recipe for success, either way.
I think we'll get a few European wars at some point, but it might not be a general European war on the scale of the world wars. It's hard to tell, though, as there are so many revanchist tendencies that could arise that some conflict may be impossible to avoid.
There is never a certainty when it comes to nuclear weapons either, but it is noted that, without the taboo, there will also be fewer restrictions against nuclear weapons. That directly implies that more will have them, not fewer.
The problems with colonizing Congo come more from its environment than from the population. And if population were truly the only obstacle, there are far earlier methods that could be conducted by selective breeding. And the Congo will have plenty of issues of its own, and the population will dip some, as it did in every major colony at the time. A large part will be through the introduction of diseases to the interior, and a not-insubstantial chunk will be through violence of various forms.