Change Shift key binding

In general you can change keybindings in the ingame menu -> Hotkeys. I doubt you will be able to change shift though. The behaviour of the modkeys is baked fairly deep into the engine. It is possible to do things like write a widget that swaps how modkeys modify commands, but I doubt all parts of the UI would be accessible. For a broken keyboard you may want to look into AutoHotkey.

AutoHotkey is reasonably easy to use. For your purposes, just find another key that you want to use as shift, and then make a script that contains (example is using the Windows key as the shift key, only while in Spring)

#IfWinActive ahk_exe spring.exe
AppsKey::Shift

Then either right-click and run it, or compile it into an .exe file and run that. Bonus with the .exe is that you can put it in the Start Menu/Startup folder and have it always running whenever you restart.