If I were anybody not named Walmsley, Rupp, or Korir that could reasonably run sub-2:11 on a flat course, I would let him go on the first half and hope I could reel him in on the second half. I suspect Rupp and Korir won't be hurt by an early pace, and I think Walmsley has no choice but to set an early blazing pace if he wants a shot at making the team (i.e. I don't think he's going to close faster than some of the other guys who can run <2:11 on a flat course).
If Walmsley goes from the gun, it will be a real test for that 2nd pack to trust they can reel in the first pack AND outkick their peers running next to them. I predict one or two guys will probably not have the patience and try to go with the first pack - having a slow, miserable last 10k. For me Korir is an interesting question mark - his fast marathon was flat. Will the late hills challenge him, or is he strong enough that he will overcome? If the former is true, Walmsley maybe has an outside shot at holding off whoever is closing hard from the 2nd pack. However, I suspect Rupp and possibly Korir will gap him in that last 10k. So, if one person catches him from the second pack, he has no chance - and I think this is the most likely outcome.