[Drumstix42]

I dunno about the target switching clearing memory for useabilityonplayer. That's a specific targeted spellcast, but I haven't tested enough to know...
However, it doesn't seem to matter much now with what I figured out -- as you can press the buttons twice now with no problem, as there's no target switching required.

Also, the /target_previous had trouble with normal user spell cast followed by a button spell cast, as I had explained above.

Again though, taking out the need of /target_previous was key and removing the absorbed input of the player/groupwindow while pressing the button fixes any problems.