I know its possible to integrate NPAPI plugins with QTWebKit as its been supported since the release of QT 4.5.
My question is, should I go and design my plugin according to the Mozilla/Gecko documentaion -Which is probably the only available/reliable documentation for NPAPI beside some really old book called "Programming NetScape Plug-ins"- or does QT handle NPAPI plugins in a different way?
Going with Mozillas and other documentation like colonelpanics tutorial will be fine. You might also want to take a look at FireBreaths source because we already solved some common issues there.
QtWebKit actually wraps WebKit and i don't recall there being any real differences between Mozilla and WebKit. Of course you should handle possible differences in supported browser-properties etc., but you should do that in any NPAPI plugin anyway.
The added benefit is that you won't have too much problems using the same plugin in other enviroments then QtWebKit.