The Alt key (mostly mapped to the Mod1 - modifier) is used not only by Inkscape but also by many many many .. many other programs: Gimp, Blender, Browsers, Editors, etc. And not only on Unix but also on Windows and Mac. Thus, I personally use the Win key as the trigger for most
OnWindow Mod4 Mouse1 :StartMoving
OnWindow Mod4 Mouse3 :StartResizing NearestCorner
(part of my
.fluxbox/keys). You could use some undocumented actions to switch to a different
KeyMode when the focus switched to Inkscape and switches back to the default behavior when Inkscape loses the focus but in the long term I think that will only confuse your user experience. Use the Win - key and be happy with it.