Is the problem that REAPER does not recognize your mousewheel, or that shift-mousewheel is assigned to the wrong action in the MIDI editor?
If shift-mousewheel works fine when shifting CC positions, it is probably not a driver issue.
In the Action list's MIDI editor section, try assigning shift+mousewheel to the action "View: Scroll horizontally (MIDI relative/mousewheel)".
REAPER provides separate actions and shortcuts for different parts of the interface such as the Main section (arrange view) and the MIDI editor section. By dividing actions into sections, REAPER allows you to use the same shortcut for different actions in different sections, so that you don't run out of shortcut keys.
For example, when working in the main Arrange window, you can use "S" to split items, and while working in the MIDI editor, you can use "S" to split notes.
But this also means that when you customize the scroll shortcut in the Main section, you have to do the same in the MIDI editor section too, if you prefer to keep the shortcuts consistent.
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