Though the code sucked ass.
Anyways, I am currently making an automoderation bot for Lemmy. You might struggle to find good documentation for the API though.
Mbin/Kbin is pretty nice, though it lacks mobile apps. I would switch to it if it had a few mobile apps, but we only have interstellar, which is pretty much alpha.
The best way is to read the docs, read code and experiment yourself.
I'll try to answer them, though:
It's hosted on instance A and B. Every instance creates a local copy.
Your account gets deleted. If you want, you also can select to delete your content (images, posts, comments, etc.). But since this is the Fediverse, it is not guaranteed that your account and content is deleted everywhere.
Iirc they do
The existence of the instance itself won't increase load, but the users on it can.
There is no karma counter builtin, but it is also not something hard to implement. But I remember an chromium/firefox extension displaying it.
"Karma" in Lemmy can be calculated manually though.
You can moderate remote communities, but you can't create ones. Though you can "own" (be the top mod) in one iirc.