Matrix (not Matrix.org please)
Fijxu
Yes. I host my own using Mailu.io. With the proper records, you will be able to send emails to any big email provider (proton, gmail, outlook). You need to pick a good TLD (.com, .net, .org, etc) so you don't get your email thrown into the spam folder immediately.
If you buy a domain now, you will probably get on the Spamhaus blacklist, which every big email service seems to use (again, proton, gmail, outlook, and probably others), so you will need to wait a few months and keep a good spam record (well, don't send spam emails obviously and keep your email server with the proper configurations).
Also, pick a good VPS provider (No vultr, no linode) with low levels of abuse, because if you setup your email server in an IP range with a lot of abusers, you may get your email flagged. (You can check that using https://www.uceprotect.net/en/rblcheck.php, but I'm not sure if uceprotect is trustable).
I think so? It's pretty solid and it works well overall. Anything is better than Google or Bing for your online privacy. If you want more privacy, you can try using SearxNG or 4get
I think its better to leave it Closed Source, otherwise, YouTube team will be able to see how it works and how they circumvent their efforts on blocking 3rd party clients.