Grace notes...aren't acting like grace notes.

Be sure to use the grace note with the slash through it if you want that quickly-crushed effect. A grace note without the slash is actually supposed to be played slowly - see http://en.wikipedia.org/wiki/Ornament_(music)#Appoggiatura. But it is true that MuseScore doesn't even crush the accIaccatura type of grace note (the one with the slash through it) if the note it precedes is long. That's already been submitted as an issue - http://musescore.org/en/node/11090. Hopefully the next version will change this.

Meanwhile you need to get a specific acciaccatura to play shorter, you could try playing with the Note Properties (right click menu). If you can't get what you want by adjusting the time offsets, you could simply silence the notes and then enter your desired playback in another voice, and mark those notes invisible.