For the not getting dirty part, I wear looser fitting sweat pants and a light coat or thin long sleeve shirt over my good clothes. This keeps me dirt and hair free.
I also carry a small bottle of perfume in the car, lotion, and baby wipes. I can wipe down my face, hands and whatever else needs it. Lotion up my hands and arms if they need it and then apply a tiny amount of perfume. I’ve never had anyone say anything and I’ve flat out asked a few people if I smelt funny, no one noticed if I did.
If you have to wear makeup carry a small travel sized kit and apply it before you leave the barn, but try to keep it simple if you do so it doesn’t take too much time, and if you use waterproof mascara just apply before you ride.
Also, if you do self care prep everything ahead of time on the weekends, have hay nets filled and ready to go, a spare set of clean empty buckets so you don’t have to scrub dirty ones until later, and as suggested groom and blanket or sheet up your horse overnight to keep him clean. Also you can get away with only grooming the areas where the tack sits and do a better grooming later on.
As for the light, all of the suggestions are great. I like small solar lights or those that mount on fence posts. You could also bring a tiny generator and a portable light system with you if you have the ability to do it.