If you look at Jenson’s body language during the last six races and his race performances, then it’s pretty obvious that he’d resigned himself to retirement.
Then there’s this (paraphrased) quote: “We waited until after the contract (reserve/sabbatical for 2017/18) was signed before Jenson was shown any of the design for the 2017 car”. For him to jump ship to Merc would be a conspiracy theory too far. He’s a placeholder at McLaren, partly to help develop the car and partly as an insurance policy against Fernando walking mid-season. Don’t forget that there have been two seasons on the trot that McLaren have had to draft in the reserve guy.
But, I’d really, really like to be proven incorrect.
Smart money says Bottas to Merc. He’s a proven talent, the right age to stay with the team for some years and his character is calm and measured, in contract to Hamilton’s somewhat mercurial nature. The last thing Merc wants is a partnership akin to Mansell and Piquet (like Hamilton and Alonso/Vettel for example).
As for the vacant Williams seat, they need an experienced hand, crucially with alcohol sponsorship requiring a more mature driver than Stroll and Wehrlein/Ocon. That leaves Massa (with no contract commitments for 2017) or Button (tied to McLaren for the foreseeable).