If you don’t use the account at the same time, would it be possible for two or more people to use the same paid account (without duo)?
Up to 10 devices can use one Proton VPN paid account. Who those 10 people / devices are, is up to you.
Yes, if you trust another person to potentially access your account data.
How? You login to the VPN app with your credentials, but that doesn’t auto log in to anything else.
Your entire Proton account is behind the same credentials. Mail, Pass, Drive etc and the personal data in your settings
(You can add a secondary password for each app to prevent it)