SkyRanger mentioned a thread back or so that OSR will likely cause people to reach the ABomb problem much earlier than usual due to the way it works with ticks, but I assumed that meant that the counter would go up faster, not that you'd meet the problem faster than the counter counted. In any case, if you're using OSR you should be keeping a closer eye on your ABomb than you used to need to. Of course, it may be completely urnelated to OSR in this specific case.
Well that would explain everything then. I wonder why it only affected horses though. Usually the Abomb thing messes up fires, flames, those firefly things, and a few other things that would have given it away. The irony is that my usual testing guy has the most time under OSR out of all my characters. I guess I spend too much time modding this game

I noticed the horseman stop and turns tword any nearby dead bodies when they are suppose to be galloping full speed to their destination, that is a check box right? I will undo this in the horse and rider tabs in the CS and see if that solve that.
That's the corpse check flag on the horses. The UOP fixes that.
Rider often seems to lose their horse if I am not in the same cell. I see them ruining up the road instead of riding their horse. Back tracking I see no dead horse, I see NO horse at all as if it did not enable with the parent rider.
I see that on occasion too, but they're usually legion patrols who dismounted at some point to chase something into the woods. More often than not though, I come upon them partway through their route and they're mounting the horse in order to continue on to their destination. I don't know of anything that fixes this particular issue.
Will not stay on the road, often deviates into short cuts through the hills instead of following the blue grid path.
Almost certainly the road record issue. Blue path nodes are meaningless over long distances. If an NPC is pathing to anywhere that would require you to load the cell into memory, it uses the underlying ROAD record to do so instead. The vanilla record is terrible and often leads to NPCs wandering well off the roads, and in Skingrad they'll even go so far as to leap from the castle bridge. The UOP also fixes this issue as of 3.2.0, with further refinement of the record in the UOP Supplementals.