I'll take these a little out of order:
It was an option Lincoln turned down. Thirty years later....
Never mind Lincoln, it was turned down by the Royal Navy in the Napoleonic Wars when Thomas Cochrane advocated it (rather presciently because it would open the can of worms of the enemy doing the same in return). Although obviously that was rather less sophisticated than what we would think of now as gas warfare. He had another go at trying to get them to adopt it for the Crimean War.
As Admiral Matt says, Lincoln was presented with a workable design for a chlorine-gas shell, and chemical warfare was considered realistic enough in the 1890s that the Hague Convention explicitly banned it. I hadn't been aware of the earlier instances that Thande mentions, but I'm not surprised by them either. In any event, the delivery systems used in World War I in OTL were simple enough that they could easily have been duplicated with 1890s technology. Gas shells appeared within the first year of our Great War, and I'm using approximately the same timetable for this one.
I once described TTL's Great War as WW1 with machine guns, trenches and poison gas but without armor or aircraft. I've since been persuaded that primitive aircraft (both heavier and lighter than air), as well as proto-armor vehicles such as self-propelled artillery or armored troop carriers, might be possible by the end of the war, although not with sufficient numbers or sophistication to be a game-changer like armor was in OTL's war. Gas won't be a game-changer either - it's unreliable (what happens when the wind changes?) and relatively easy to defend against.
One thing that probably
will appear in significant numbers by the third year is trucks, which won't directly affect combat but will affect logistics profoundly, especially in places where horses can't go.
Also, Mikoyan from Eritrea should be in Yemen proper, not Hadramut, especially because it's there that the Zaydi clans are. The area would be perfect to exploit dissent, but would be under Ottoman, not Omani, rule. And if the British lose Aden, the FAR get a serious advantage.
My impression is that the Za'idi Shi'ite clans were transitional between Yemen proper and Hahdramaut, and that the area was contested between the Ottomans and the Omanis although not really under anyone's firm rule. I'm happy to be proven wrong on that, though, and Mikoyan is certainly in Yemen proper at the time of the update.
BTW, Mikoyan and most of his officers are Russian, not Eritrean, although most of the troops and some of the junior officers are from Eritrea. The Ethiopian volunteers are led by Russian or Russified Eritrean officers (many of them noncoms given field promotions) and the Za'idis are led by their own tribal chiefs.
Also, I'm confused about the Russian segment. Aden is across the Red Sea from Ethiopia and Eritrea. How did the Russians get a substantial army across the Red Sea from Eritrea? Even if the British can't spare naval forces to blockade the Bab-el-Mandeb, where did the Russians get the transport to supply their forces? Are they living off the land?
The RN doesn't have the resources to blockade the entire Eritrean coast, and Mikoyan's troops crossed in small civilian craft (much of it seized from the Italians during the capture of Assab) while the Russian naval station at New Moscow/Massawa created a diversion.
You're correct that the Russians can't supply this force more than intermittently, unless and until Aden falls. They're living partly off the land and partly off the generosity of friendly Za'idi clans and Yemeni mountain chiefs, and they're hoping the provisions thus gained will last long enough to reach their objective. Mikoyan is taking a big gamble (which is why most of his force consists of local allies and foreign volunteers - the higher-ups at New Moscow weren't willing to spare any more of the Russian garrison) but he thinks the possibility of taking Aden is worth the risk. Not only is Aden a valuable prize in itself, but its seizure would divert British and Ottoman troops that are badly needed elsewhere, and might move Ethiopia closer to joining the war.
A quarter of North Germany's territory under occupation? Ouch.
Yes, but how much of that is actually valuable land like the Ruhr or Silesia, and how much of it is comparatively less valuable land, like East Prussia?
iirc the European borders are essentially 'frozen' as of 1869, so you can look at this
IOTL map of the NGC to get an idea of where things stand. The North Germans are probably holding the French at the Rhine, but the south German states allied to the FARs the French are likely across the Rhön and through the Thuringian Forest, fighting their way up the Weser trying to cut off the Prussians western front. The Austrians are fighting through Silesia as per this update. The Russians I'd assume have are at least to the Vistula, if not across it. Which all paints a fairly grim picture for the BOGs, but not a horrible one.
At least in the East, the Vistula seems like a good place for an impassable defensive line.
Lower Vistula, maybe. Middle Vistula bisects Congress Poland, and Upper makes for a border between A-H. and Russia. Considering where Russians attacked in OTL WWI and adding how the railways run, the most likely defensive line goes from Danzig up the Vistula to Bydgoszcz then along Notec do Krzyz then south towards Oder river and Breslau. It would also make most of Greater Poland in Russian hands.
In the east, the Russians have been stopped between the Vistula and the Warthe (albeit with some exceptions - Danzig and Königsberg are still in North German hands and are being resupplied by sea). The lines have actually reached the Warthe at Posen, although they're closer to the Vistula further north.
The Austrians have advanced into Saxony, and both the Russians and the Austrians have pushed into Silesia, although these advances have been stalled by terrain and by exceptionally strong North German defenses (the North Germans know very well that if they lose their industry, they lose the war). The Russians are stalled east of Breslau although they've reached the upper Oder at some points; the Austrians have failed to take Dresden but are threatening to cut it off.
Wolf_brother's description of the positions on the western and southern fronts is correct. The Ruhr is safe for now - the French tried to push a salient north, but were stopped well short of the industrial region.
If/when the BOG counter-attack comes, I wonder if Wilhelm II might try to copy his namesake and push the North Germans across the Ores into Bohemia - it'd be extremely risky, but the PR power of it alone might appeal to him (remember; ITTL the Seven Weeks War was the last major one the Prussians can claim to have won).
That's certainly the kind of thing Wilhelm II (who is an ATL sibling, but who has more or less the same personality as the Wilhelm we know and love) would want to do. He won't be content to let the FARs bleed themselves to death against the North German defensive lines, and will be looking for ways to attack.
How would the Sudeten Germans receive him at this point in time - were they loyal to the Habsburgs, or would they be more likely to support Prussian pan-Germanism?
I have a feeling based on this latest installment that France is going to push through Belgium, violating their neutrality in return for some slight advantage. Also, if they are worrying about Indians arriving in North German trenches, why not try to arm some revolutionary elements in the sub-continent?
Leclair isn't quite ready for that yet - he's still hoping that he can negotiate transit rights, and he realizes that if the French army has to fight its way through Belgium, the North Germans will be ready by the time it reaches the border. What's going to happen in the near term is a great deal of French-sponsored agitation in Belgian politics - there will be many French francs in the ultramontanes' coffers.
Arming Indian revolutionaries will certainly be done, although it will mostly be the Russians doing it. The French might also try leafletting the Indian troops, not so much to cause a mutiny as to make the British officer corps believe that there is a
risk of mutiny and undermine their trust in the Indian soldiers.
Hmm, how is the Philippines faring in this TL?
Still Spanish, but not very content with that state of affairs - TTL's Spain is rather more liberal than OTL's, but its reforms to colonial government haven't been enough to satisfy the Filipino nationalists.
