Michael,

Yes, the left swipe only works when Metro apps are running..

I don't imagine a left touch would interfere with the left windows app switching..

I do like your idea for the left thumb modifier or menu. Would love to try it..

Its only a matter of time before we have utilities to customize multitouch gestures that function across the board or within a given app.. I wonder how multitouch will play out.. With everyone implementing multitouch we could end up with the tower of Babel.