And those are also very good questions since the PIN thing was new and new things could indicate being hacked, and people changing their public key is a thing that Signal does seem to try to de-emphasize as a strong security signal so I'm almost more impressed they noticed it and were concerned.
It sounds like your parents are very unusually observant! I have had coworkers much younger than that who cannot be bothered to read messages like that.
My parents (and my extended family + friends) have been using signal since the Snowden leaks.
All I had to do was ask :) It's not really a hard app to use either, I had a few calls from them when they introduced the PIN stuff and when I or someone else have switched phone but it's not comprehensible. My parents are closing in on 80 now.