Have to say I don’t like the idea of outlets in the stall at only 5ft tall – you really truly won’t use those outlets every day, and I think the protruding plugs will be too attractive for itchy faces, curious mouths. For bucket heaters, run those all on a dedicated circuit so you can switch them off all at once rather than unplug.
ETA- maybe better solution is to have a built-in small step for you, under the outlets. Could be as simple as cutting a 12" length from a 6" or 8" fencepost and secure it to the wall. It’s nothing that the horse would get hurt on. Or get creative and have a fold-down step attached to the wall.
Re: putting switches near barn entrance: if there’s more than one entrance into your barn, make it a 3-way switch so that you can turn the lights on/off from either entrance.
Re: individual stall light switches vs. all together: Only situation I can think of where you’d really want only to light one stall would be a night check for an injured horse or broodmare, and you feel bad about waking them all up. I think I’d get over the “feeling bad” part (it’s not like they have to get up early for work in the morning :lol:). But one idea would be to run them on a single switch/circuit, BUT have switches on each stall also. Default position would be for all the stalls to be switched on, so they all go on from the main switch. When you have those rare cases as above, you could turn all the other stall switches to Off, so they don’t light up when you flip the main switch. Might be overengineering what is not a big problem to begin with, though.
Use covered outlets for the ones by your troughs. I don’t have that and regret it.