[MPNumB]

Yes. If player uses a button, than two use functions should be called:
1. Player using the button.
2. Button using a door or a camera or whatever.

FM_Use will not hook the first one. In other words FM_Use isn't called when player uses something with +use button, that's why I think it's best to block the key or the search of entities near by. Also for example func_pushable object isn't called neither by FM_Use, neither by Ham_Use (though here I may be wrong, but all I'm sure that returning HAM_SUPERCEDE inside Ham_Use for pushable objects doesn't block player from grabbing and later moving the object).