I have booked a LHR-LAX itin with a stop in NYC to see a friend. There is less than 12 hrs in NYC but I fly in to LGA and out from JFK (the connections from IAD worked better that way).

MP won't let me use one SWU for the whole journey because the computer thinks LGA to JFK is an open jaw. After half an hour on the phone, and several checks with a supervisor and the rates desk, the CSR said the best they could do was reroute me LGA-ORD-LAX. This would cost an extra £75 and lose the PS flight out of JFK. Changing to IAD-JFK would cost much more.

Is it worth trying again? There seem to be ample precedents that UA think NYC is one place. For example, I have no problems booking fares which don't permit stopovers but do include a LGA/JFK transfer, which would suggest that the two are considered a single destination. Furthermore, there are plenty of fares rules which make it clear that JFK/LGA/EWR is treated as the same place.