I think I understand your situation. Am I correct is assuming that the great C fare yr getting is only valid from JFK, and connecting through ORD is not an option? It seems this is how you'd be mostly conveniently routed to Asia on UA. You can have UA link your pnr's but it doesn't make much difference. UA can easily check your baggage from SYR-final destination. US could not since you change airports!

Taking the trip to IAD sounds like a nightmare. Any little thing could happen there that could seriously screw with your trip. Trust me, you'll be pulling your hair out when you realize you are SOL over night at Dulles after your transpacific flight. The 7:30pm flight time is extra malicious, as you'd most likely connect to the last flight out to SYR, no doubt!

US to/from LGA doesn't sound so great either. The cab would cost around $30 I'm thinking. IF your flights are on time, you'll be hard pressed to make the 7pm flight from LGA on the way home!

Have you tried JetBlue? Their morning flight gets you in at 7:20am. The return flight doesn't leave until 10pm. Not too convenient but for being in the right airport with out another connection.

Sorry to be negative, but no clear option is presenting itself! I'm sure others will have some useful insight.