So apparantly Shift-Scrolling does not work on many people's devices.

a) does anyone know why this is happening, it works fine for me

b) am I allowed to hotfix (as in upload a new build) new keybinds to the mechanic in order to facilitate those people playing?

I believe so, the rules don't say you're not allowed and also the uploads are still available. In game jams where you're not allowed to fix bugs and whatnot after submission deadline the upload is usually locked during the voting period.