Ouch... ya when I made the gui's for mod selector, I used windows builder. Just stay away from swt.
BTW, marinepower's proxy had a delay on nodebuffs. If you got hit with, lets say blind, your screen would be blinded and then a second later a message would pop up saying cleared and you'd no longer be blind. I think this describes the issue you are having.
So basically the client is applying the effect even before the server has a chance to say you have it. I don't think you'll be able to get around this without editing the swf.