I am not understanding why the leaking overflow is causing the sump to overflow unless the durso main drain is too low in the tank in which case the sump is too small for the amount of water when the DT back drains. This is assuming the issue of the return nozzles being to deep in the DT has been addressed. If the durso is the right height, the fact that the actual overflow is not water tight is of no concern. I have built a few tanks and currently have center overflow with two return nozzles. I set my main drain height (and the safety drain) at a level that is equal to or slightly higher than my return nozzles. I still have room in my sump, but my overflow is also water tight so it is immaterial. Here is what I would do take the new tank, clean the inside of the overflow real good with acetone (don't breath it unless you also like to "huff" or whatever it is called). You can then put a new layer of silicone on the seams. It will stick, just not as good as virgin surfaces but since this is not structural like the DT seams it will work just fine. The other thing that puzzles me is why are you getting sand in the overflow pipes? They should be well about your sand bed even if a little comes through the bad seam. Good Luck, sounds like you are overdue.
