I recently did an ex DUS MFU to JFK. Definitely the way to go. Not absolutely the cheapest but allows London stopovers and the flights are a breeze (less than an hour in the air). DUS airport is probably the finest back to back turnaround that you will ever find anywhere.

No probs so far as bags for the return are concerned. Just say you are dropping off your bags in London for your mum / wife / housekeeper to sort out your laundry (no stopover does NOT mean you cant enter the UK and spend time there - just that the limit is 24 hrs) . An alternative that I have used was to say that I had to meet someone in London and only after meeting them would I be sure which onward flight I would take. The last explanation helpfully got me offloaded from the flight on the last leg of my existing booking, which left the last leg (LHR-LIS) neatly available for a future ex LIS.

Actually, I dont imagine that many check-in agents would particularly care if you said you weren't going to take the last flight. This was only a problem when people were crowing about doing it in the dying days of Concorde. Probably best not to try thoug; just in case !