Hi Admiral!
I have some access to japanese sources via japanese friends of mine. This would be a good point to say a big thank you to Michiko, Sakako, Takumi, Hikaru and others of their friends for that. (esp. since my Japanese is not good enough for now to truly understand some of the old stuff)
Despite all up and downs over the years, there was some kind of connection between Germans and Japanese from the start. That is an important point in all that. It can function ATL because of this basic connection. It would not go the same with another nation. As there are possible other ATLs where specific event/things can start only because a basic connection the e.g. French share with their special friends or the British do.
Don´t get it the wrong way, but how do you come to the conclusions why the Germans and the Japanese do this from just the first two parts?
Yes, Schmoller proposed it and I mentioned it already in ATL text that the national governments back home have to ratify it. But that will go smooth because of the general situation, not because of great visions. The Eulenburg expedition was done by Prussia explicitly to further her ambitions in Germany and the world. Schmoller´s self-initiative gives them that, a diplomatic victory Austria will not be able to counter soon, if ever and the economic side does not hurt either. Schmoller and the very influential von Delbrück were VERY able economs and esp. can communicate to others the possibilities of this new treaty. You underestimate von Delbrück´s standing in the whole Zollverein and the Prussian was an early fan of far trade. Had the ATL situation happened OTL, von Delbrück would have championed it too, that is clear from the documents on him and his personality.
As for the medium german states, I took Baden as an example, because there are old documents existing, where landlocked Baden deeply hopes to find a way to the larger world markets. This ATL treaty gives them that, if they ratify it. They won´t be totally pleased at first, but this thing is what they yearned for their wildly budding industry.
As for Japan: Tenno Komei was no idiot. Getting rid of the Tokugawa was his main goal, but he knew that his land needed to rise fast, before the foreigners cashed them in as a colony. The ATL Germans proposal will help saving Japan from that fate. Feuds are one thing, Nippon is a different matter. And the Tenno met the german delegation in Edo, so he has some first image of them. As for the truly anti-foreigner: Mori´s reaction is just a placeholder/example how some groups will react.
As for the Zollverein: Please read the OTL treaty, if your German is up to it. The Zollverein and vassalisation are two very different kind of shoes. That the Zollverein incomes are sent to Berlin is simply because the main regulatory office is in Berlin, where the net gains for each member are calculated and then sent back to the various capitals. All member states have their representatives in Berlin.