[woodPurchase]

If they were similar in numbers why would it be the Japanese ceding instead of the foreigners? Even if so, it's all the more reason to not let them in in the first place. Keep Japan Japanese for the Japanese

[fluoridation]

It's not a battle, where one side decisively wins over the other. Both cultures cede a bit towards common ground.

[woodPurchase]

I'm not going to say that's untrue, it obviously isn't. But friction, distance, and drift are going to work in the opposite direction

The result depends on the ratio. I don't see why we can't have trade and relations with other countries without also assimilating eachother

[fluoridation]

I don't think trade and diplomatic relations is enough. Most people are neither international traders nor diplomats. You need constant cultural exchange by a large sections of the populations for the cultures to drift towards each other.