Love the battle reports. Just started reading it, but your first question (Is it worth to pay the premium for 2 regiments over a horde in Kingdoms of Men - or KoM-) is one I’ve also encountered.
Short answer: no.
Long answer: it’s complicated.
First of all, KoM love their hordes. Most are cheap cuz they don’t pay for fancy rules and they still do hit quite well with their 25 attacks. I know it’s not mSU, but an army with 4+ hordes is perfectly viable as Kingdoms of Men hordes are not expensive. (165 for shield wall or 175 for polearms). Especially since you mention unlock problems, hordes might help you out by a lot.
For instance, take an army I played a year ago (pre clash '22) . It’s 16 units for 2,300 points, one more than your 15-unit MSU lists. It features 4 hordes, one of which is a horde of knights. Even with 4 hordes, I still max out my unlocks. I play 4 artillery and 6 characters, needing those 4 hordes and 2 regiments to unlock.
If I wanted MSU, I’d drop the horde of knights for whatever cheap infantry I had laying around (preferably polearms) and spent the remaining points on more units. I find that in Kingdoms of Men, the base unit is a horde.
Our troops simply aren’t as good, with possible exceptions for bowmen and fanatics. If only our polearm troops would return…. I’d gladly field four of them! The reason for this is that we have great cheap chaff in the form of the wizard-on-pegasus for 85 points or the “premium-priced” captain on pegasus for 90. These chaff have comparable nerve (10/12) but fly and are nimble on a 50x50 base.
That said, there is a certain horde tax-. After 3 (maybe 4) hordes, these units become too unwieldy and get in each other’s way. That’s why I think a 5th horde would be wasted, and I doubt about the 4th. So IMHO, the best composition of a Kingdoms of Men army is 4 hordes and as many regiments as you can fit in. Troops of fanatics (fearless!) are a good form of chaff if you are alergic to flying horses for some reason or suffer from unlocks.