We did evening EMH at the beginning of last August and it was fine. As far as crowds go, it wasn't that crowded, but still kinda busy. But as the night went on, the crowds got thinner and thinner (it was pretty low after an hour of EMH...but especially low later in the night). The only lines we experienced were for the mountains. Those had waits of about 30 minutes...but that was only at the beginning of EMH. As the night went on, the wait became five minutes and less since less people were there. It wasn't nearly as bad as I've read about...and it certainly not that bad if you stick it out the entire time because by the end, there's hardly anyone there.
As far as how late it goes, it all depends on what time the park closes. Evening EMH is 3 hours long...so just tack on 3 hours to the closing time of the park and that's how late it will be.