Should probably mention that premium is only 10 bucks a year. I also don't just pay for the feature itself but also to support Bitwarden, it's completely free and open source after all.
I keep google authenticator around just to store bitwarden's totp. But I also store bitwarden's totp inside bitwarden, so I can use bitwarden's mobile client to get bitwarden's totp when I log into bitwarden on another device.
Nothing in particular, all my totp was in google authenticator and over the years I migrated them all to lastpass then bitwarden, and the only thing left there is now bitwarden totp.
Kinda think its to keep costs down. They already have their builtin totp generators so making a separate application seems kinda redundant. Unless you mean why they don't make their own hardware security key? That's probably also to keep costs down (materials, vendors, marketing, upkeep, etc...). I would also like to have a rival with the same credibility for yubikey though in case something happens to the company.
No. If anyone has access to your email or master password, they can simply reset any other account. How would your difficult (one time used) password of protonmail be leaked? Proton doesn't have it. Only if you've got powerful malware on your device and then it doesn't matter in which app your shit is stored.