Good news: the federation problem on fedia.io, as well as the problem with getting an error 500 when submitting a thread, have been solved.
Fedia.io is sort of like she Ship of Theseus right now - I literally replaced nearly everything trying to get it back working.
The problem ended up being a silent out of memory error that php-fpm was running into. I had to increase the memory limit to about 10x what the docs require to get it to work, but once I did that, it works great.
I was only able to sort this out after @bentigorlich recommended I move the site to debug mode (which requires me to lock everyone else out). Once I did that, it started giving some useful errors.
My apologies for the amount of time it took to fix this. I learned a lot about php today.
