On the reserve army idea...that's...effectively what the Army of the Forest is already doing by regularly marching up and down the province keeping an eye on all the villages? If you actually formed a fourth army, it would require quite the pump-up of pay (in that you'd be paying them rather than maintaining their equipment deliveries and such) and equipment normalizing. They're militia troops, they're not going to simply rebel when Frederick dies, Magnus has regularly fought amongst all 3 armies of Ostland at one time or another, who themselves coordinate with various militia when nearby all the time. So does Frederick, but less so due to duties keeping him locked up more often. There's not that much cross-village communication anyway, at least amongst militia. The numbers are one thing, actual coordination and connection are entirely another. Plus, you're really going to need to give a convincing argument to somehow draw in militia 'elites' who don't want to serve in the Army of Ostland proper, go try to join the Bull Warriors, become a mercenary elsewhere, go on an adventure outside the province, etc. Like, the truly exceptional militia members, if/when they crop up, have several avenues available to them already to pursue. Plus, army numbers upkeep has already been established previously in the thread, with a variety of things (Shallyans, Jade Wizards, etc.) keeping up against chip damage per turn. A lot of militia people won't want to leave home as well unless absolutely ordered to, so that's an issue too.
Like, on the issue of loyalty...that's just the breaks in this universe. You simply cannot guarantee 100% loyalty for every single militia member to any one person or another. Plenty of them are in fact probably more immediately loyal to their local lord than the Elector Count who they'll most likely never see in their entire lives. But I can guarantee that they'll be a lot more loyal to Magnus than you apparently seem to think once Frederick finally kicks the bucket. Will it be the exact same levels to Frederick? No. But neither will things plunge into the depths of heresy and depravity the moment he ceases to be. Magnus is trusted, blooded, well-known, and popular amongst the province. I don't know what else I can say, honestly.