My playthroughs have apparently been very different (including my 1.4.2), except for Sturgia - they always lose. However, who they lose too seems to be different each playthrough (current playthrough - I've conquered half the map; Vlad, Batt, and Kuz hit them hard; but let them keep 2 towns at the top middle. Older playthrough kuz usually just ran through all their towns, etc.). I agree, Sturgia needs "something" more.
For me, it always starts with everyone always attacking Batt, Sturg, and Aseri first; then the empire internal fights - empire "winner" changes each playthrough; at about this point, I'm ready to join kingdom/create my own. The only "winning" kingdom in any playthrough was Kuz. But then again, history says they conquered the "world", so it somewhat makes sense. Not that I want that to be a "thing" ingame... I almost wish the top kingdom would change every playthrough.
(sidenote: I think this has a lot to do with merc/minor factions. 2 playthroughs ago, Kuz had 4 or 5 of the minor factions helping - no idea why. Kuz were dominating, and I thought minor factions were suppose to help the "weak" ones? Maybe Sturgia should auto-hire more minor factions?)
Every culture has "horse" troops, so I don't see why anything needs a nerf? I don't know the details of each horse troop, but overall they all have horse units, so it appears balanced to me. Maybe other kingdoms need to prioritize horse units more?
The only weird thing with declaring war for me (1.4.2), is that super weak kingdoms are declaring war on me. Ex. I have a kingdom with 52000 strength, and north and asari still declare war on me (and both are under 2000 strength). Not sure that makes too much sense... they have 1 town and 1 castle, little militia; and I could probably solo there fiefs with no worries (I make peace quickly, since I want them to live).
I also find doing the "main" quest (dragon banner), causes way more wars. Making your own kingdom "feels" safers.
I personally haven't executed in a long time, because of all the negative side effects. I release them all for charm exp. I haven't had an issue yet. Needless to say, I'm ok with the negative side effects of execution. Maybe they need to add more repercussions - i.e. any friend of the executed automatically declares war and multiple kingdoms come attacking - "you think you can kill my friend?!" The game designers are basically saying that executing is not the "best" way to go about business, but give you options to play how you want. I think it is good the way it is, but I'm open to changes too.
Them returning the next day to capture a fief, was patched a while back (i think current beta has a 5-6 day grace period, and they get less starting troops?). That would also indicate you haven't garrisoned your fief if someone quickly tries to take it back. Hell, even my companion armies (110-170 size) will capture a fief if the other kingdom didn't protect it. It was awesome seeing an opposing kingdom freshly capture a castle, only to get a notification minutes later that one of my companions just captured it for me