If a Kbin member requests deletion of their personal account and they happen to be a community owner, would ownership of that community default to the moderator with the next-longest tenure? That's how it worked at the bad place, is it the same way here?

Not sure, but account deletion is a manual process here. I suspect what actually happens is that the magazine is tranferred to the default owner / first admin account. On kbin.social that would be ernest.

See for example https://kbin.social/m/trans - a sub with few threads. I think the original owner successfully requested account deletion which is why that sub is owned by ernest now.

See also https://kbin.social/m/kbinMeta/t/258090/How-does-Delete-Account-work-currently

How long does account deletion normally take after the deletion is requested?

Not sure of the historical average time. It's a manual process though so it will take some time for the admins to get to it.

Also, do the posts get nuked along with the account, or do they remain on Kbin?

I saw an example of this some months ago. It seems like the posts do get nuked, though with recent updates I'm not 100% certain that this is still the case. Again see /m/trans - most likely it was one of those subs where most of the threads were started by the owner posting, so when the owner's account was deleted, so to did those threads and posts.

Actually it's worse than this - as the entire thread is gone, including other commenters' replies.