I really think it'll be Wehrlein. They've been angling to get him in for a while, need to show faith in their young driver program, and it'll likely save them a lot of hassles as I think Lewis will have a consistent edge. If Mercedes still have a sizeable performance advantage, it'll be easy to put all their eggs in the one basket.
I hope I'm wrong - the Alonso idea is the most interesting to me (and Wolf has spoken of him being an option to replace their drivers in the past) - but I think it's much easier all round for Mercedes to go with their home grown guy.