Maybe Mitchell is clear in his own mind that he wants to join Argyle and has been told that depending on the deal the club manages to do to bring a keeper in they will be able to review the financial package they are able to offer him. Maybe he's quite understanding of that situation, and so happy to sit and wait to see how things unfold.
Charlton can't force him to move to another club (permanently or on loan), but if he doesn't move they are left having to pay his wages for another season. He could always return to Argyle on loan, and if he isn't in Charlton's plans, then they are hardly going to refuse to loan him to us and turn down the opportunity to claw back some of the money they will have to pay him in the process.
If Mitchell really wants to come back to play at Home Park then I reckon he'll be able to one way or another. I think everything depends on just how much he wants to be based down here.