the Cert just confirms, that the domain your accessing is belonging to who owns it. When you signup for a cert at LetsEncrypt, you have to run a script on the source, which confirms as your domain.
You wouldn't be able to get a Cert for e.g. amazon.com - because you wouldn't be able to run that specific script on the source and so LetsEncrypt couldn't confirm if that domain really is yours or not. And that's as well the reason, why not trust everyone,
I use it since 2016, but with my own domain, so can't really speak for the "@ proton" domains
But I never had trouble accessing it in a foreign country, it worked flawless
True.
Then with Proton (idk about tutanota) you should be able to mail E2EE with them, since you can import their Public Keys into Proton. It's not just "E2EE Proton2Proton", since Proton uses PGP, you can safely mail E2EE with anyone using PGP.
D‘you have any source for that? Sounds kinda way too good somehow for me ^^