Looks good - shame the protectionist Americans are taxing your business interests to the hilt (maybe it's because Honest Abe has been dead for a few years at this time?

).
From reading this and other AARs, it seems that the businessmen possess staggering sums of money to make up for their utter lack of sense. Shown by such genius acts as importing things that are produced in abundance domestically, for about five times their domestic cost. Do they ever bring in anything good?
Actually the last time I checked, Honest Abe was still President of the United States, might have changed though.
The merchantmen on many occasions bring joyous things like Gems and Gold, which I get a staggering amount of taxes (excise) from... my state funds are usually pretty healthy since I produce silk domestically.
They also sometimes bring me 'common goods' of things I don't get, or the market is too small to get a piece of, like sugar and rum which helps keep my contentment high.
really like the care you are taking to set this out and how things are developing. Re diplomacy, I'd push the options they can't refuse (support, access etc) and trade with them, try to get things to +25, then push for commercial agreements and state visits - at the moment I suspect you are going to get knocked back everytime you ask.
keep using the sewer option, it both increases development level and population growth so its a really good option in your most populous provinces
the 'tax your businessmen' is permanent, in effect its whatever rate they tax domestic production.
as to the random imports - yeah some are pretty dim but on balance for a power that can't afford to trade aggressively for luxuries they are a useful tool to obtain some ... & in turn that helps boost domestic contentment
I'm usually a fan of gold, gems, rum and sugar...
I do plan to 'spam' the sewer option on Edo after the Boshin war, for now I'm just developing the southern portions of Japan since I'm confident they won't switch to the rebels.