Yesterday I had the chance to ask a SME on the flow of foriegn fighters from the UK to Syria and whcih faction they joined:
Of the estimated 800 who have gone out to fight, half went to the non-jihadist groups in the early stages and after the taking of Mosul everyone has gone to join ISIS.
Most UK estimates are that 300 have returned, with a substantial number not being detected by the authorities.

Personally I doubt if any have escaped from ISIS. I would expect expressing doubts would be seen as a threat and they would be killed.