I use mailbox.org personally. Disroot is probably fine. Do they have 2FA? That would be the most essential thing you want here if you’re worried about being hacked by an outside party. 2FA would even mitigate a password leak in most cases, since they’d only have 1 of the authentication factors.
If you’re worried about hacking, you can do some things to mitigate the damage that would cause. Download important old emails and delete them from the server, this is pretty easy to do in a desktop client (like thunderbird or outlook) where you’d just move them to a local folder. That way if someone gains access, or they sell to someone that processes the data, they won’t have the old emails (unless they for some reason retained a separate copy, which seems doubtful).
Sign your email up for https://haveibeenpwned.com/. Then you’ll get notifications if there’s any data leaks, including of your email provider. Obviously this is only useful if nobody has stolen your account before the leak is reported, but that’s more likely than not (unless you’re a particularly valuable target for some reason).
Yea, I couldn’t tell you the specifics. I know new members of group chats don’t see any previous messages. I think it might re-negotiate the keys every time someone is added. It’s probably not meant to scale up to very large groups (tho I’ve never tried), but I’ve noticed no issues in 25ish people chats.
For DF: The free version had a Linux build for a long time. The paid version adds new graphics, and it took a while for that to get a Linux release.
For Minecraft: you should be able to play without an account if you’re single player and using a third party launcher. I almost exclusively play with friends.