I never actually erased his memory, merely planted a false one of the Sorcerer heading towards the palace. But yeah, the false memory is permanent unless he begins to suspect his mind has been screwed with.
So, with your permission, I would like to subject Cal-Doran to a bit of torture, Sith poisons (Dark Side corrupting kind, thus creating more inner demons) and a small memory rub for purposes of misdirection.
It would be interesting, given that Sin'ryk's behaviour would be considered suspicious for those well-versed in suspicion. But are you sure you want Cal-Doran to face a Sith Master?