Yes, he gets Rheinsberg back. (Though Heinrich is going to miss it, he loved that place as much as Fritz did.) Mind you, while I do think retirement could turn out to be the best thing ever to hoppen to Fritz at this point, finally freeing him from his self chosen galley, I would like to point out that MT's offer is only her public plan. Don't forget, in secret, she's sent the Chevalier d'Eon on a rescue mission, and of course if the Chevalier does manage to rescue Joseph, then everything is off the table.

If we don't want to make Fritz too stubborn, he can even agree to the prisoner's exchange, but then before it can happen, in the eleventh hour the Chevalier rescues Joseph and Fritz is back in Austrian captivity. Though MT mmight remember that offer of Young King FW/Regent Heinrich + Retired For Good To Rheinsberg Fritz = Peace once Voltaire's campaign really takes off and she decides this whole Fritz on trial thing is just more trouble than it's worth, still a bad precedent, and did that scoundrel Voltaire really just threaten to publish a trashy tell all about her court, co-starring beloved husband's mistresses (painful, but has happened once or twice before) and beloved daughter-in-law carrying on with beloved daughter (no way!)? (Seydlitz picked something up from Isabella's reaction and told Voltaire.)