You can render option #2 above impossible by never using or accessing ProtonMail in any way that could tie your account to your real identity.
True anonymity is a real rabbit hole. But for starters you would access it through an anonymizing protocol like Tor; only use a free account (or pay with Bitcoin); and never give out your ProtonMail address in any context where anyone (including your ISP or anyone listening "on the wire") could connect the address to your real name.
To their credit, ProtonMail does seem to be doing everything they can on their end to make anonymous use of their service possible. They claim that they don't log "IPs used to connect to accounts, or the times certain accounts are accessed"; and crucially they make it possible to use the service without using a credit card.