Yes you can stain / dye both sides of the leather. The grain side / smooth side will always look the best though. I'm not sure how thick the leather your talking about using is. I don't understand MM's to well and i'm to tired to look for conversion chart...LOL. If you glue two pieces topgeather the botton pice needs to be cut a little wider. Glue both togeather and then trim, you may need to sand the edges some depending how well you cut the extra off. I don't make and leashes for sale but the ones i have made for my dog i used one piece of leather. He didn't care. But to resell finished on both sides could be used as a selling point.
I use Angelus dyes and really like them. The dyes are nice bright colors for the most part. I use red, blue, burgundy, ox blood, green, purple, and pink sometimes. I like them a lot better than Febings dye. The acrylic's are paint i think. Not something i would use to color a whole leash with. I have never used a air brush so i can't say about that. Their are people on here that do use airbrushes though. Maybe one of them will answer that question for you.