Not sure about the Darts but the Dolphins I am told have delicate seals. Even the slightest running dry can ruin a seal.
This is a predicament. Had no idea the sump couldn't be drilled. If you are stuck with the tank and can't return it this might pose a problem.
Don't know if anyone has ever tried it but My thoughts would be to have a piece of glass cut for a divider. Have a hole drilled in the divider. Seal the divider in the tank and put a bulkhead in the divider. Then you have a dry chamber in the sump for the pump to sit in, and it will NEVER loose prime. No risk whatsoever to the shaft seal, this is of course assuming they are sensative like the Dolphins, IF I am even being informed correctly about the Dolphin sseals......
I would pass your idea by the manufacturer, see what they think. I really think the divider is the solution though.
I apologize if you feel I lead you astray. I'm sure in the long run you will be VERY happy with the pump though.