I think there is a slight change in assumptions. With Plasma MVP, the user can delegate the task of watching the chain and initiating withdrawals to someone else, so the user does not actually have to be online regularly. The delegate could grief the user by initiating unnecessary withdrawals, or could fail to do their job, but could not steal the user’s coins.
In Plasma XT, it seems like a user could not delegate the checkpointing task while they are offline to an untrusted entity, because they would have to provide their private key to that entity for the purpose of signing off on checkpoints.
Do you see a solution that would enable checkpoints to be made while the user is offline, or would they just have to allow the coin history to grow and then make a new checkpoint when they do eventually come online?