I thought Clothing saved on the Horse was bugged too - because I would end up with the same outfit from Slot 1 when I tried to change to Slot 2, etc. What actually is the case is that it takes several seconds to change - so Arthur does the shrugging/shake animation in his clothes, nothing changes and you think the wrong outfit is in the wrong slot, but really, if you wait for several seconds after the animation finishes, the correct outfit will appear on Arthur. It takes just long enough that I was going "What the heck - nothing is happening! That same outfit was in Slot 2?!" And I'd go back to the Clothing option on the Horse menu - rinse and repeat.
I think this is a loading issue, because it only seems to happen if you haven't swapped outfits recently. Once the new outfit is loaded, you'll find you can change back to the old one almost instantly and it occurs during the animation instead of after.
I would LOVE it if they let us give custom names to Custom Outfit Slots, so it isn't just Slot 1, Slot 2, etc. and instead we could label them something, like "Crime", "Fancy", "Hunting", etc.
I agree on the off-hand holster positioning. I don't care for it either, and would prefer to see Arthur wearing it in the traditional way. But yeah, it's probably because of the satchel, so Rockstar isn't going to change it.