How can I use ProtonVPN on Android without using my ProtonMail password?

Correct, per security measures on proton’s website. It does this by default to protect your data.

how tech savy are your family members? Forensic scientists?

It is not relevant. I always assume everyone is a forensic scientist. See, I don’t think it’s wise to reason security away. I don’t want to have to argue that my housemates should be trusted and may not be skilled enough. Such reasoning may have be part of a “prevent breach” mindset. I’m following the latest security advisories like “assume breach” and “zero trust principle”.

Apart from that, thank you for the tips.