I bet it was fun... Not sure how to get around that.
Well, you can leave the upper wall open (there's always ambient light around outside, even at night with the power off). The alternative is to install those funny little emergency light boxes with the two floods on them. They'll run off batteries long enough to dismount and walk the horse out!
There is a guy out here that builds barns that are add-ons. Let me see if I can describe it well enough. Have you see the stalls/pipe corrals at the feed stores, 12X12 made out of pipe with plywood sides and metal skin and roof? It is similar to that, although the front of the roof extends six feet. You can put up one, add another across from it and you have two stalls and a covered alley. You just add to it as you need or can afford, with each new stall needing only three sides rather than four.
I don't see why you couldn't start like that, however, instead of joining up the stalls overhead, space them far enough apart to have the center space you need. The barns go up sort of like legos. My barn, which isn't that type but the principal is the same, had prebuilt sections that were put together.
You can solve the problem of lights out in the indoor arena by installing some emergency lighting that automatically goes on when the power goes out.
My concern with the tack rooms between stalls is that chemicals may end up in them and in case of a fire, they would go like torches.