There are a lot of good questions here. I will answer them one by one. But first I do have to say that alternate history is fiction and necessarily requires imagination.
The British mostly recognized the Southern rebels for economic reasons (remember in 1861 it looked as though neither side was capable of forcing the other to surrender and bu 1862 the rebels seemed capable of holding out) and war in the Americas was just as bad for the British as it was for the US. The British certainly didn't like the blockade but submitted to it (officially, but yes ship yards built blockade runners and warships, something they had done for ages and would definitely not stop doing just because they were asked, Palmerston had no say in that).
[They did eventually get around to seizing the Laird rams after a direct threat of war conveyed by the US ambassador. And fitting out warships was against the Queen's neutrality proclamation. They actually had to take the CSS Alabama to St. Helena (or the Azores?) to couple it with its guns. A very large monetary settlement was later paid after arbitration in Geneva.)
Was there probably some idea of vengeance for recognizing the rebels in 1830? Probably, but the British government at the time saw this as another proof showing the weakness of the Republican system and recognized the other side as a belligerent in the hopes of mediating the conflict, not in the hopes of overtly supporting the Confederacy.
(Perhaps. I will have to go back to Amanda Foreman's World on Fire and one other book I read on US/UK ACW era relations. But the effect was to open the UK's worldwide system of coaling stations and ports to the nascent CSN. The CSS Alabama was welcomed with fanfare into port in Cape Town and in Australia. And yes, I would not doubt that resentment for recognition of the Canadian rebels in the 1830's played a part.)
Yes but even here the Fenians will not have enough men to do as you are proposing. I'm not quite sure how many men the Fenians had available to them in real life (something like 1500 if I'm correct?) but even twice that number is not enough to seriously inconvenience the British once reinforcements arrive.
(10K men were across the river in Buffalo, prevented from crossing by the gunboat USS Michigan, the only warship on the demilitarized lakes. About 800 made it into Canada with no artillery. 150K Irishmen fought for the Union with 7 Irish generals.)
Remember, the early Fenian victories were against poorly trained and disorganized Canadian militia (who still managed to give them some hurt) and then they retreated in the face of British regulars. Not the force I would be hedging my bets on.
Once they come into contact with British regulars my prediction sits firmly with the British for victory.
(True, the militia did well facing off against experienced veterans and through overwhelming numbers were able to push them back. ITTL, the invasion force will be the entire 10K with artillery and sufficient supplies. In winter, Ontario and Quebec cannot be reinforced without US acquiescence. Having the Grand Trunk end in Portland and a canal around rapids on the St. Lawrence be on the US side did not show great foresight. I read a whole book on preparations for war in New Brunswick. Even though the governor general did his best, militia was hardly strong enough to resist an invasion.)
If the Fenians are attacking in winter they are extra doomed. The Canadian winter is not known for its niceness and attempting to march a large force across the frontier and supply it is a hazardous proposition at best. They can dig in all they like but they suffer greatly, and the British know this and let them wither on the vine while massing troops in the Maritimes and shipping them up on sleds or slowly moving their forces into proper winter quarters while waiting to crush the poor Fenians in the spring.
(This is true and I have to do some more research on winter fighting. In the BROS thread, the writer stated that winter battles did take place and winter warfare, while difficult, was possible. With open rail lines in the rear and perhaps a small city or village to shelter in, resupply and billeting would not seem to be so difficult. There is a long debate on the BROS thread about how many troops could be quickly dispatched to Canada and how many local militia could be mustered. I think the author makes a good case for his calculations there so I will not dive into it here.)
The British could easily repeat the mobilization they had undertaken from the Trent affair. That alone is going to crush the Fenians, especially now that the British have modern arms stockpiled in the aftermath.
(The mobilization during the Trent affair was how many men? 10K?)
It is important to remember the British are unlikely to be blindsided by this. They will know the Americans are supporting the Fenians (through spies or through rumor and hard to hide preparations) and will supply, garrison, fortify the frontier accordingly.
(Now this is past true. The British were very good at spying and made their protests to the US gov. but still managed to be somewhat blindsided by the 1866 invasion. The Fenians boasted of their plans in newspapers but still the frontier was not secure. The real deterrent to a successful invasion was US enforcement of neutrality.)
Ship troops across the Atlantic, quarter them in the Maritimes, ship them up river on sleds or very slowly march them between winter quarters awaiting the thaw? Fairly easy.
(Maybe. I have not gotten to this question in my research yet.)
Not much of a conundrum? They can protest Sewards actions and lodge formal diplomatic complaints, respond by covertly supporting Maximillian and Nappy III in Mexico to distract the US, and first and fore mostly crush the Fenians making it a moot point.
(Well, formal diplomatic complaints only go so far. They were passed back and forth during the ACW. The British were in on the invasion of Mexico in the beginning but withdrew early. See my comments on Grant's insistence on expelling the French from Mexico and Seward's finesse of the situation above. Note that the possibility of this contingency is good reason to keep reserve units actively drilling.)
Remember, the Fenian raids solidified Canadian and British ties and Canadian identities. Making them worse only helps that.
(True. The Act of Union did come just one year later and partly in response to the Fenian threat. I have to further study the 1830 and 1840's era rebellions in Canada including the annexation proclamation. Note that I have eschewed, as I think L would have, an outright conquest. Just breaking the ties between the UK and Canada or splintering Canada is a US victory.)
The British behavior in the ACW was not particularly atrocious, nor was it against the grain for how European powers behaved for centuries. Supplying arms and warships to a belligerent power is the prerogative of British manufacturers, not Her Majesty's Government. Lincoln and Seward both know this and attempting to punish the British is in poor taste, not to mention economically and politically dicey. There wasn't exactly a pent up bottle of anti-British feeling in the US post-ACW.)
(And this is just the American posture. What are you complaining about?, the question is posed. Is this not how things work? For warships, the UK government did forbid it. I have to look further and see about small arms. The seizure of the Laird rams was an extrajudicial act, without any sort of due process before the fact, done in the face of a threat of war. Without recognition as belligerents, the CSN would not have been resupplying in British ports, tended by British coal tenders... And exactly, citizens may do as they will. How much more so if by UK law they are not American citizens at all but UK citizens? The UK cannot have it both ways. And by this logic, if US officers on leave man ships or lead men, they are just acting in their private capacity? No?
In 1860, I would say no, there was not a great deal of anti-British feeling. One of the princes visited the US to great fanfare and a warm welcome. But St. Albans, the Trent Affair... There were open clamorings in the newspapers for war. And keep in mind, ITTL the operations of the CSA's Canada cabinet and the presence of Booth prior to his assassination attempt come to light.)
Not only would this be 100% different (seriously, harboring an invading army, supplying them, and officially recognizing them? Not what Britain did at all) and the British would know it. It was one thing for British private citizens to do the things as discussed, but it's another for the whole US government to do it!
(Not really. The UK did just that to a smaller extent, manufacturing and supplying warships, financing the rebels. There is nothing more ITTL than recognition as belligerents, just what the UK did.)
[I have to go for now but will finish up later. Good questions all though some of them I have addressed before. Thanks for helping me flesh out my TL. Cheers.)
I doubt Lincoln would have done much differently personally. Increased financial support to the rebels, increasing arms shipments, increased diplomatic pressure on the French. Why risk an unpopular war?
I admit Lincoln was a great politician, but this is unlikely.
This is a blatant act of war. Not to mention the British aren't stupid.
I don't quite follow here. The Americans are blatantly starting a civil war in this scenario. That wouldn't be fooling anyone.
1) I'm skeptical. Who is financing this and why? the US certainly isn't going to foot the bill to bankroll a Fenian army and navy in the wake of a civil war. Nor do I suspect they would be willing to keep the money flowing if the Fenians face defeat (try selling that to the people at home)
2) The British were more neutral in the ACW than you give them credit for. Here the Americans are basically being the political equivalent of rabid dogs.
3) Ok this is absurd. These are men who have just fought a long conflict against each other. What incentive do they have to suddenly join a cause that is not their own, to fight alongside one another against a power that has done them no wrong, and in what is a blatant invasion of a foreign land?
Where is this untapped well of seething anti-British sentiment, where is the pent up rage against the Queen that Northerners and Southerners are suddenly willing to put aside 4 years of warfare against one another to go and fight Great Britain?
This is blatant aggression by the US, this is absolutely not the same thing GB did in the ACW.
A few major questions:
1) Why do L and S think this is a good idea? If the US was fine settling the post-war hostilities with diplomacy and legal procedures why do L and S (already tired from war) look to go starting another one? They had political capital, not that much!
Also why jeopardize trade with your largest trading partner? It makes no sense in the aftermath of the war with the British being eager to get back to business as usual.
2) How large are you actually envisioning this Fenian force being? Anything less than 50,000 is far far to little to meet even the what the US thought would be necessary to conquer/occupy Canada successfully.
If it is 50,000 where do the men come from? There aren't that many radical Irishmen in the US, and there certainly aren't that many sincerely anti-British people within a years gathering effort in the Union. This addresses my 'where is this supposed wellspring of anti-British sentiment' question.
3) Why do the British in this scenario seem so oblivious? There's not a chance in hell they don't catch a slight whiff of this coming and don't pre-plan accordingly.
Not taking a minuscule force of Irish radicals seriously is one thing. Not seeing the US trying to intentionally bolster a larger one is another entirely.
L and S are acting far too belligerent and almost cartoonishly evil here for something they gain with far less effort easier in OTL.