A couple of thoughts/questions. Is there any reason the stall cannot be pushed into the corner? I agree that getting in that small area is tough to access hay and feed.
I would only think you need water in one place, not 2. Maybe attach a water spigot somewhere accessible to the cross tie are, so that gives you water close to the stall as well as for washing the horse. Not too close tho, unless you have a floor drain. (esp for winter). I would avoid putting it on outside walls, personally due to freezing more easily.
Don't forget an electrical outlet near your grooming area for clippers or whatever.
If you use a sliding type door on the tack room you will be able to use the wall on the way into the tack room for storage (perhaps metal trash cans for feed?).
A shelf in the tack area with a small microwave is great-for warm water in winter, etc.
I also do not like the post coming right out of the stall. Perhaps one of the posts for the cross ties could be the actual corner post for the stall?
You should be able to add a window in the crosstie area-next to the door-but I would also think about one on the side (20') wall in addition. Good cross ventilation, as well as light. And, if you shove the stall back into the corner, you won't need that window, unless you want a small one in the stall for ventilation on those yukky winter days just for a bit of air.......
Signature undergoing edits. Please standby.......