Technical question 1: as an alternative to a full block, is there a way to “soft block”, or tag, or whatever, some people, to act as a sort of CW?

Not ideal, but Sync for Lemmy premium (ultra) has a custom tagging/highlighting feature. Not sure about other apps. It’s subscription or a large lifetime payment. But there is cloud syncing (and other features).

Which phone?

I don’t even see how it’s possible to stop someone from using social media until they’re 18 unless they truly don’t want to.

You badly underestimate the proportion of people who have never heard of Mastadon.

RIP Mario and Luigi

Oh you’re in for a treat. I’d really recommend playing the original Paper Mario first if you haven’t already.