99% leaves the same residue as lower concentrations of iso. However, it is much more efficient.

Denatured alcohol is usually less than 10% methanol. I wouldn't use it regularly for hand cleaner, but the toxicity is pretty low.

Isoppropyl potentially has more BTUs than ethyl and methyl. But you can't get all them buggers unless your stove combusts it fully. The carbon residue indicates that you have incomplete combustion. Therefore, you are not getting the full benefit of isopropyl. I know of only a few simple stoves that can achieve complete combustion with iso. And they are not very good for other reasons, IMHO.