I'm a Walmsley fan but don't think he'll crack the top three at the Trials, though I do believe top 5-10 is possible if everything goes perfectly for him.
However, there are repeated mischaracterizations here that need rebutting. First, his buildup to his 1:04 half-marathon in Houston in early 2019 was abbreviated (just 6-7 weeks, go look at his Strava log), and the first training block like it he had done with any kind of sustained intervals/tempos targeted at those distances in years.
You don't get to PR half-marathon or marathon shape in your very first training cycle aimed at it, and one of insufficient length to boot. It takes at least a couple cycles, and maybe more like three or more before you're going to have properly developed the body's energy systems relevant to those distances enough for a true PR-level effort. So, in my opinion, the 1:04 is not a good indicator of his ability at that distance or for the marathon.
The performance equivalency charts rate a 1:04 half at something like a 2:13-2:14 marathon. If you take a look at Walmsey's collegiate PRs for 4:04 in the mile and 13:52 in the 5,000, those equate to roughly the same, but those were prior to full physical maturity and on reduced mileage. (He was coached by Juli Benson at the time who does not focus on mileage, and Walmsley has indicated she prevented him from running the miles he would have preferred.) As someone above pointed out, Fauble's coach (Ben Rosario, at NAU in Flagstaff, there where Walmsley is based) believes Walmsley was 2:11-capable at the time Fauble was running 2:12.
Perhaps he would theoretically be capable of a little better at this point due to continued development, maybe 2:10, I would guess. But that's only if all systems are go and he does a longer and better-targeted training block for the Trials than he did for the Houston half. And it's also assuming just a second training block within a year can get him to that level, which might be possible, but not probable. Maybe if everything goes perfectly, it miiiiiight, but that's a big ask.
I do think an advantage Walmsley has going for him is the megamiles and hill work he does. But I also believe that kind of huge base will only come into play if he does exactly the right kind of sustained work during this current/upcoming training properly targeting intervals and tempos at the right paces and amounts. I don't think the fact the base miles are at slower pace are a detriment, given the terrain and altitude he trains at (he's usually at 7,000-feet-plus, sometimes 8 to 9,000).
In fact, I think those kind of parameters are an advantage because they make it more doable to sustain megamileage like that in the first place, by reducing the chances for injury. (Slower pace, and far more varied terrain leading to less overuse-prone movement.) Again, though, I don't think megamileage done in this way will bear fruit at the half and full marathon distances unless he is shrewd about how he balances that with intervals/tempos.
Whether he succeeds or not at the Trials, however that's defined (top 5-10 would be that, in my book, top 3 out of this world), it's still a very interesting experiment and story, and good for interest in both sports.