People have been turning a blind eye to what their allies have been doing since forever. There's nothing new there, nor surprising. Any discontent over Melhed's actions will only manifest against them when more convenient to do so.
As for all of these remarks about the odds... Power is the relation of work over time. That's how it is in physics, and I think it applies perfectly to armies. Wiki's example is that a kg of coal has more power than kg of TNT, because of the time over which work is exercised.
Having a total mobile CS of 20000 is nice, sure, much nicer than having a total mobile CS of 5000. But if it takes a week and a half for the first realm to get to the front line, and just as much to get back, then that's 20000 CS of might applied per three weeks. Wheras if it takes 3 days to defend the front line, and another 4 to refit, then the latter realm has 5000 CS over one week. So it's more like 6666 CSw vs 5000 CSw. Yes, the first realm is still noticeably more powerful, but no longer as much, having a 6.6:5 ratio instead of 4:1 ratio. This is because the marching army will accumulate soldier pay and equipment damage before even entering battle, and even a victory is likely to cause problematic equipment damage. The much smaller army, by avoiding the incoming enemy or by siding with militia, can often keep this army in check long enough to force a refit. Far-away armies rarely have much time to loot or do takeovers.
Of course, if all of the armies came to Thalmarkin simultaneously, it would pose a problem. However, organizing such feat is incredibly problematic on its own, and rarely optimal or safe. Enweil managed to fend off almost every single realm of the continent simultaneously for a very long period. Not because it was stronger than all of them combined, it obviously wasn't. Not because all of the enemies were so far off, most were not. But because they weren't coordinated. One army came, we'd defeat them, and then a few days later the next army would come. We were stronger than any one of them alone, and alone is how they came at us.
The odds aren't all that skewed against Thalmarkin.