Mercedes thought they could handle the "let them race" mentality, and has found that as a top team, it is not always a great idea. Its just another reason why I think Wehrlein (newly crowned ""Autosport Rookie of the year") is going to get the nod.
Vettel has supposedly ruled himself out -
http://www.mirror.co.uk/sport/formula-1/sebastian-vettel-denies-could-man-9393592 (although, if I were him, and considering it, I would certainly keep it secret from Ferrari, and be publicly in denial!)
Wolff himself stated that his primary options are a number 2 driver, and promoting internally.
Pairing Alonso with Hamilton is just too risky.
Button - I really don't think he is up for it. Seems very happy to be getting on with his non-driving role. I am just surprised he has not ruled himself out publicly yet.
Merc have testing in late feb, and will have a simulator program to run. Wehrlein is already experienced with processes of the car. He knows the engineers, and most recently did the tyre test for Merc. This gives him more time to get on with it than other drivers.
I fully agree that had this been known about earlier in the year, then Wehrlein may have been overlooked again for someone else on the grid, but I think needs must. Mercedes is a germany company, and I think they will always be inherently conservative and Wehrlein is the conservative choice.
Wehrlein or Bottas are my bets on replacement.
Wolff already said he does not like that idea.
Toto Wolff said:
"It’s not my intention to interfere with Ferrari’s internal issues.
“And I don’t even know if Sebastian really has the desire to come to us.
“What would Ferrari and McLaren do without Vettel and Alonso in December? Or Williams without [Valtteri] Bottas?
“The third path I mentioned is the one I like least.”
The Third path being, poaching a top driver from someone else.