As for the private demiplane: Forbiddance will block anyone from entering this demiplane - probably even a Silver Key.

Here's the trick to not be jailed there yourself: leave a certain spot out of the Forbiddance area and cover it with a big rock, when you are inside or even better set up a Forcecage and Antimagic Field traps there (Anticipate Teleport might be a good idea as well). Have some loyal minions there to either put the traps on hold, when you go back from adventuring or bail you out if you had no time to tell them. Idealy you never leave your demiplane anyway, since there are Simulacra and Astral Projection is just that good. So anyone wanting to break into yout extraplanar stronghold would have to anticipate the moment, when you send someone out of your demiplane or the moment, when one of your simulacra or other minions returns. The thing is, the wizard chooses the moment himself, so he will be prepared and on his own turf.