IMO, if McDermott cannot get one of his top 2 candidates for OC, whoever they may be, then he should consider retaining ALynn assuming he doesn't get a HC gig elsewhere and is agreeable.
As for ALynn, if his goal is to be a HC and he doesn't get a shot this year, the quickest route will be immediate success as an OC. He knows the Bills players, he's a run first guy, and running a watered down GRo offense he lead the league in rushing, was third in turnovers, and finished just outside the top 10 in scoring with limited receiving help much of the season. He should know what he can do here, already, and with a little tweaking and some luck, could realistically improve on it in 2017. Going someplace else and starting from scratch is more of a gamble, IMO.
Both of these points are made with the assumption that Tyrod stays and that the team isn't in a total rebuild mode. If either one of these things is not the case, then I think McDermott should move on from ALynn.