I agree that speech should be there from the start, and that Abe would lose a lot of his uniqueness if he were to lose the stitches altogether. I would however love to see some actual development of his abilities after he escapes RF.
How about just explaining his stitches stretched as he got older? If accepted continuity is that Sam sewed his mouth shut to stop him crying as a baby, then it follows the stitches were only intended to be temporary until he learnt to keep his mouth shut! Maybe Sam used a stretchy material knowing as he got bigger and stronger, he would eventually be able to talk?
As far as his abilities evolving, have Abe learn possession once he reaches the Monsaic lines. Any mudokon can talk, so it's natural Abe can from the beginning. I think the biggest debate is how he knows spiritual stuff without being exposed to it. Learning chanting from the natives would really show that Abe's journey taught him something and that he changed as a result.
In fact, possession is not explained in game until Monsaic lines anyway. Most (if not all) of the rescues pre-escape rely on gamespeak, mines, levers and boulders to kill sligs anyway. In my opinion, we can keep all of the early puzzle and gamespeak learning from the beginning in place, and perhaps expand on the possession element at the start of the Monsaic lines.
Abe is supposed to feel like the little weak guy sneaking out of RF, then gaining power and going Rambo on his way back in! I think a greater emphasis on hiding from sligs during the escape would add loads to the tension of being caught. We simply do not need possession right at the beginning
