I just don't believe he'll be back. Keep in mind that the the 2018 cars will be very different from the current model. With a full year of development to the 2017 regs that lack of knowledge will seriously hamper his efforts of getting on terms with Alonso.
Of course Fernando will be talking up Jenson because if he gets pushed all the way by Vandoorne in 17 he's hardly likely to want nore of the same the following year.
If Vandoorne has a season comparable to Hamilton's competitiveness against Alonso in 07, then could it be the end of Alonso, at McLaren at least? Either way, Jenson wouldn't want to get back into a car and get smoked by Vandoorne as well.
I guess there is a precedent, set by Prost in 1992 but with all due respect to F1's loveliest boy, he does not have the credibility to walk back into a front running team.