Summary: Shift should make things go horizontally. At least Gimp and Firefox agree on this.
Your Alt means
vertical scrolling for the mouse wheel, but
horizontal movement for the dragging. Sounds more inconsistent than necessary.
The fundamental ideas are sound, let's see what other programs do.
Google Maps needs Ctrl+Wheel to zoom, and does nothing on wheel alone, even complains to the user that you should press Ctrl+Wheel to zoom. Probably because it's usually embedded in a webpage, and webpages have their rules for mousewheel. The restriction of being embedded in a browser trumps all other concerns, thus let's ignore Google Maps.
In Gimp:
- Ctrl+Wheel zooms.
- Wheel (without modifier keys) scrolls vertically.
- Shift+Wheel scrolls horizontally.
- Alt+Wheel seems to do nothing, but I'm not sure if this even gets through xfce4 to Gimp, see below.
In xfce4, my window manager, Alt+Wheel zooms the entire desktop, i.e., all windows become larger and I have to scroll around my desktop by moving the mouse. For me, this is a really dumb feature, but maybe it's useful for the sight-impaired. Anyway, I never accidentally press Alt+Wheel and it never annoys me.
It's possible that xfce4 eats the Alt+Wheel entirely and doesn't pass it on to Gimp. I haven't dug into the xfce4 config to remap Alt+Wheel.
In Firefox:
- Ctrl+Wheel zooms.
- Wheel (without modifier keys) scrolls vertically.
- Shift+Wheel scrolls horizontally.
- Alt+Wheel again seems to do nothing, but I can't test for sure.
Thus, at least between Gimp and Firefox, we have consistency: Shift always means horizontal action.
-- Simon