All of these are “root” mounts. I don’t explicitly mount any subvolumes (they get “implicitly” mounted as folders though)

It’s possible by having the webfinger endpoints at the “root” while keeping the rest of Lemmy on a subdomain. The main thing that determines the domain in your username is webfinger.

No clue if Lemmy or kbin support this config though, but quite a bit of the microblog-only parts of fedi do, and it’s a widely used thing.

On a desktop or especially laptop case, it should be equal to (or larger than) your RAM if you use hibernation (as RAM gets copied to swap during hibernation)

On my server, I set it up to be 2GBs, mostly arbitrarily. Right now it’s at 500MB, but my main memory is also only 600-800MB full out of the total 4GBs available, so I’m not running out of RAM anytime soon.

Swap behavior seems to have changed a while ago, so consider reading https://chrisdown.name/2018/01/02/in-defence-of-swap.html on how it works right now. Hell, even that might be outdated nowadays. Up to date info on how swap works really seems hard to come by.