Where did you find the info that U of Montana has the least subsidized FCS program? I don't doubt it, but that seems like a recipe for success at a higher level, not an inherent risk. They have a very loyal alumni base and a pretty sizeable endowment for an FCS school, outpacing more than half of the Group of 5 conference schools. It's a small school, but has a large loyal following and draws big crowds for FCS, ranking 2nd in the entire FCS in attendance (Montana State was 3rd last year as well). Both Montana schools appear to be in the realm of being ready to go for something bigger as a package deal.
If Montana were a MWC school right now, they'd have the 3rd best attendance (After Fresno and Boise), Montana State would have the 8th best in the conference (out of 13, if they were in it). So they are right there and have nothing to lose except a bunch of money to make it happen. They are in a WAY better position to make that move than Idaho was when they did. WAY better.
That said, Texas State is still probably the best overall bet for the Pac. UNT, UTSA could be good as well, really locking down 3 very large Texas schools, creating a 10 team conference for Football. That said, if none of those work out, if FCS schools are wanting to come up, the Montana Schools are easily the best equipped to do so for the Pac and the cost to the conference, if they were to help make it happen, isn't really all that much compared to the MWC fees.