Here's basically all of the hotkeys locked out of heroes the moment you start making one: MSHAPO High Priest hero has a spell hotkey of S (an early skill too)
Not sure about any others
OT: So if you have multiple units selected and not just DSum, if you click spells they don't work until you have just him selected? Bug on your machine, doesn't do that for me. Mod proofed don't want any exploit abuse.