Currently "loot all" when in combat will cause you to cast whatever spell is bound to that button instead. Maybe add an option for the loot window to take priority?

Due to WoW's restrictions in combat, we can't selectively propagate or consume a key press. The alternative for this case is the game will not respond to any key other E (the current default hotkey), meaning you won't be able to move with WASD or cast any spell until the loot window is closed.

I see, that sucks, as this means you can't really rely on it as your primary way of looting; you don't always immediately drop combat after a fight ends (it can linger for a second), meaning you can easily try to loot too fast and end up just closing the window instead. Also, you can't efficiently loot mid-combat, which is particularly annoying if you're doing some sort of group farm, or if you're unlucky enough to aggro a mob while e.g. mining.

I could imagine disabling all inputs until the loot window is closed would be too restrictive and cause more annoyance than it's worth. Might be best to just keep auto loot on, and use "Loot all" as a backup.

You can now change the hotkey for Take All. If you bind it to a modifier key (SHIFT/ALT/CTRL), you won't worry about casting the spell bound to that key. It's unconventional but it's the only solution due to game restrictions.