LonelyNematocyst

On reddit fitgirls site is never mentioned in full for good reason.

That's just false. There's a direct link in the r/piracy megathread.

That's a problem solved by libreddit/redlib.

After googling, I think that was a joke about an unrelated app.

Lutris doesn't, but you can do firejail --net=none lutris

OBS supports capturing audio from a specific application, so if you set that up you don't need to be careful with foreign sounds.

Staying on your home instance is currently an issue on the repo. Implementing it yourself would need using the search, which is complicated because it only works through a websocket connection.