Emails that ends with .onion can only receive emails from other Tor mail providers. That means if your email ended with .onion, your friends using gmail or any regular mail provider won't be able to contact you. That's not in protonmail interests, they prefer a user friendly, privacy-focused email service, compatible with all the others.
Some onion mail provider like tormail offer a proxy system that makes your address accessible from the outside, but it's not in protonmail design. Proxies are here to protect the identity of the mail server, it's already known where protonmail servers are anyway.
Regarding your second question, it depends, but .onion email are considered much safer. To put it simply, if the website where you putted an .onion email get seized, they can't really do anything about this email, they aren't sure where the mail server is since it's behind Tor (assuming it's a decent onion email provider).
Whereas if you used a .com (or any clearnet domain) email, they know the email provider and can try a "visit". I've never heard of protonmail bending to authorities, and I don't think they will, but it's a risk. And even if they don't, I bet the traffic around there servers is monitored, especially non-encrypted incoming emails (yes, protonmail encrypt your inbox, but if someone send an unencrypted email over an non-encrypted connection to them, they can only encrypt it after receiving it. Who knows what happened in between.)
HOWEVER, most onion email provider don't encrypt your inbox. You have to trust them they won't read your email. Depending on what kind of usage you're planning to have, this could be a huge deal breaker. I which case, using protonmail or tutanota over Tor is your best option.