The button problem is quite common, and is due to the fact that the faceplate deforms slightly when screwed down tight, thus causing the clearance around the buttons to become less than required.
I believe that the problem is caused by the plastic supports on the rear of the moulding not being in quite the right place, thus allowing some parts of the moulding to be pulled closer to the front of the Empeg than others, resulting in a slight bowing of the faceplate.
As you've already mentioned, loosening the screws cures this problem as it allows the panel to become flat again as the tension is released.
One solution is to use some threadlock on the screws to keep them in the ideal position. My Empeg was delivered with exactly this fix already in place.
I think that later Empegs don't suffer from the problem quite as badly, due to a slightly modified moulding.
Andy.