You can probably use an Elgato Facecam (Mk.2), configure it in a Windows VM and then save the settings directly on the device to use it under Linux.
Does this support other targets like twitch VODs?
Never had an issue getting an iso from them via Mullvad.
Mattermost does not have E2EE to my knowledge.
Not E2EE though, or at least not fully.
XMPP only does it with certain client extensions. And Matrix only does it when the rooms are set up this way. SimpleX does what you want, but is kind of unintuitive for the average user.
I say go with Signal, it does what you want and is idiot-proof.
But as a third party you can not know which clients are using this outdated http mirror. On BitTorrent you can see every participating peers and some of them are probably enduser machines (depending on the actual implementation of OP's suggestion).
How do I join the closed beta?
I didn't have much luck with Bottles in the past, but this is entirely anecdotal.
You can also run firefox -p in the terminal.