Hi!
@Chris Triangle
Yes, most irredentism is not grounded much on logic and problems like you describe could pop up. In the case of Sachalin, this is different in that there were Russians, which were definite rivals of Japan. And that Tokio would try to secure the perimeter against nations with great ambitions in East Asia, is equally clear. Still, I think Germany or another nation with her focus outside the Pacific cuold slide by that problem.
In "Eisen, Blut..." German-Japanese relations already change during the Eulenburg expedition with Japan getting an offer Tokio can be sure to not get from any other Great Power out there. So the ATL Japanese are sure of Prussia/Germany´s positive intentions early on.
If we go to an ATL closer to the OTL situation, let´s say Bismarck is very intent on rallying all support for the Kulturkampf to get a full "win", that he buys Ezo even for the higher price the early Meji government set.
Now Germany has Ezo and Bismarck follows his idea of a joint development. This is part hard calculation by the chancellor, part genuine sympathy for the Japanese due to the first impression from the Eulenburg expedition, the later developments and the Takeuchi delegation. This would be a different general treatment which should convince the Japanese of the German intentions and more with Germany´s focus still solidly on Europe, Tokio should be sure that German Ezo is no danger to them, but potential help. Russian designs on Japan would definitely endanger German Ezo, so Berlin would be interested in keeping Japan upright.
The "Ferne Gefährten" book describes the German-Japanese relations over the last 150 years and even back then there was enough basic sympathy. Not only in that book, but other sources and my experiences with the Japanese show me that Germans and Japanese share some basic outlooks the Japanese do not or only partly with other Europeans, which are the switch why a German Hokkaido should function long-term with not much friction.
With Ezo already a long time part of Germany when Willi II comes into power, this would force him to keep course there, no matter his personal paranoia. And who knows, his alter ego in that TL might not have it, due to the closer contact and hence more clear situation.