Had the exact same experience with the river rock paint -- after the second coat, it still looked pretty bad because of the roller marks. We had a little extra paint, but not enough for a third coat, though it sounds as if it wouldn't have helped anyway.
Anyway, we ended up using a natural sponge to sponge on the extra paint over the entire wall. The end result isn't the smooth color like we originally intended, but the result was still VERY nice and turned what we thought was an expensive ruined paint job into one that gets a TON of positive comments. It looks great!