The MWC will likely have nearly $200 million in exit fees to distribute. I assume Air Force would likely want their share of that money before they bolt. They already play Army/Navy every year so no real reason to rush.
If the American conference is holding together, then the PAC will go after Utah State first as their 7th members, then offer UNLV the final 8th spot in hopes that gives them enough cover the breakaway from Nevada-Reno.
That would put the PAC12 at 8 teams:
Boise St, Colorado St, Fresno St, Oregon St, San Diego St, UNLV, Utah St, and Washington St
Then the Mountain West would be down to 5 full members plus Hawaii football only: Air Force, Nevada-Reno, New Mexico, San Jose St, Wyoming, and Hawaii football
They need to add 3 full members, 2 of which must have football.
UTEP and New Mexico St are the obvious candidates but the MWC has been dismissive of them for the last 20 years so they may think they are “too good” for them. But then again, beggars can’t be choosers.
FCS Sacramento State University has been lobbying for a spot as they want to move to FBS. I also assume (if Utah St is truly gone) that WAC FCS schools Utah Tech (formerly Dixie St) and Southern Utah might get a look too.
There might be a non football school like Utah Valley which could be added to offset Hawaii if they want to keep their membership/footprint tight.