The friend I'm trying to help was just someone,
   nearby-in-the-social-graph on Tumblr, who asked for help with securing
   communication and stuff.
   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...".