Their specific situation is that they're about 15, autistic and bipolar, and have both a very active and emotionally important online life, and very invasive parents who wouldn't think much before cutting that off if they thought it might help make them "easier to deal with"-like. I don't really know -- I just offered to help.

This question was trying as much for specific for them, as general strategies that minors can use -- I'm also thinking of one person from a while ago, whom I knew better and who is thankfully in a much better situation right now, who was using their abusive dad's laptop when available and terrified of the possible and likely consequences if their dad found the thread that they were getting sympathy, advice, and support in. I'm pretty sure I've run into other kids who were concerned about stuff like that. One friend is working at this problem, partially to use, partially just to get a sense of infosec, partially just in case. After I posted the original post here, another friend specifically asked me to summarize what I learned from this thread, 'cause they expect to run into people who need advice.

Mostly I'd like to be able to work out something that, if I run into someone in a similar situation, I could say "Here, try this, this, and this. Look for these things -- you can do that like this -- and if you find this then you have to take this other precaution. And if you can, try reading this and this and see if that's applicable to your situation." At least as a start. And I'd like be able to share that like I can share "well here's how you make it harder for Google/The Government/An Identity Thief to get your info...".