that again would imply molecular dispersal. what Nightcrawler does is "basically" walking through a doorway, then coming back through that doorway. he's not disappearing. he's "walking" through a doorway.
I don't want to get into an exchange, but that analogy you just utilized perfectly explains how it's possible for Nightcrawler to sever off someones arm or leg. If he only takes a piece of something/someone through his "doorway" and shuts it on them, the dimensional aperture severs the piece brought into the other dimension off from the whole, like someone's arm getting caught in a heavy security door and being severed off when it closes.