"why did the developers of amule decide to disable the amule gui when you preview a file?" <- because for some reason, the movie player is forked instead of execved (this means that the movie playing process is executed as oart of the same process, instead of being run as a separate proccess). I asked on RC1 to not have it forked, but no one has coded that yet :-/ Just aks for it not to be forked... and I guess someone will hear ;-P
"Solution: killall -9 mplayer should give you access to amule back" <- Wrong. Well, not completely worng, but partially. That will work, but that's the BAD way. If you close MPlayer by clicking the X (close) button, for soem weird reason, one of the mplayer processes remains opened (I reported that bug to mplayer six months ago now... no answer). The solution is to close it by typing 'Q' when the player window is focused. That'll cleanly quit MPlayer, and aMule GUI will be available from the very first moment ;-)
Greetings!