Proton explicitly enabled keeping 2 free accounts on the mobile apps quite some time ago, probably more than a year, so they're cool with you having 2 like that.
If you get more, you'll be hampered at the application level, but, unless it's like a load of accounts for spam purposes, having just a handful shouldn't get you banned, I believe
Selling is outside the scope of the licence, you can do whatever you want with monetisation, be it free or paid-for.
But any one person that uses your GPL if local, AGPL if local or through a remote service, has the right to request you a copy of the code and you have an obligation to comply and provide it
Must be gay or European!