I've waited 'til the last minute to put on rub before, the flavor didn't soak in as much but, could still taste it. Also, have used mustard before rubbing and don't really notice a difference. I've bought spices before that had been sitting on the store shelf for too long and weren't very flavorful. You don't have to use sauce. It's a personal preference and the proper rubbing will add plenty of flavor.
My guess would be that you didn't use enough rub or that you over spritzed, washing off the rub before it had time to set or the spices were old. Try using more rub, let it sit overnight. Let the meat smoke for 30-60 minutes before spritzing. Taste the spices straight out of the shaker, to check for freshness.
As for the sandy texture, my guess would be that you forgot to rinse the ribs or they got contaminated some how (dirty grates, maybe). Could coarse salt cause this problem? Hard to say, I've never had that problem.
Good luck and don't give up, sounds like your on the right track. My first ribs were tough and dried out.