Waiting for the final descent is too late. There is no way Evans can make up enough time on Wiggins to take a lead, much less a large enough lead to survive the final TT.

If I were BMC, I'd make the move on the Tourmalet descent. There are no flats beyond that point, so there is a reasonable chance that a breakaway could stay clear. And Evans would have 3 descents to gain time on Wiggins instead of just 1.

Getting free of Wiggins will be tough, but IMHO it would to worth trying.