The fact that you can (apparently) say meQ or meQmoH to mean the same thing doesn't mean that one necessarily arises from a dropping or adding of -moH for brevity or pragmatics. That's one possible reason, but there are others.

And we don't actually know that you can say meQmoH and mean burn (something). Maybe the existence of transitive meQ means Klingons don't accept the use of meQmoH at all, because that would be silly. I'm not saying this is the case, just that we don't know.