RE: deer decomposition
Decomposition depends on a couple of factors, a major one being weather (temp). I watched a roadkill doe last summer disappear down to a pile of bones over the course of a week. I also cut a head off of a nice buck in March that had been gutshot in December. That deer was almost completely intact.
Based on those findings, I wouldn't be overly surprised that the buck you hit could have decomp'ed to that point. The fact that the antlers are nothing but nubs makes me think it isn't though. Unless, like someone said, the buck was found by another and the antlers cut off, then chewed on. I didn't think antlers could be eatten down that much, but maybe if you have a high squirrel, porcupine, etc population it could be possible.