i wouldnt bother relapping it. as far as the rounding is concerned, its no big deal. the edges will typically round from the lapping, but the last time i checked the core sits in under the middle of the block, not the edges
you might try putting on a little more paste (you shouldnt be able to see the core under the paste).
try laying the sustem down on its side for a moment; air could be trapped inside the rad...