It's my first time trying to book a trip with some *A segments on CO.com and I'm not having much luck. Usually I book on .bomb, but this time I want the CO flight numbers ticketed for miles+copay upgrades.

I'm trying to book CO IAH-LHR connecting to LH LHR-DUS-MAN (all on a single WHW5NCE fare) and returning CO MAN-EWR-IAH (SHX6NCE fare).

If I search for a simple roundtrip IAH-MAN-IAH (with times, tried both search by price and schedule) I can get the return segments no problem, but the outbound options are all via EWR, vastly more expensive fares (+$1200), long layovers (6+ hours), or on partners (VS via MCO).

If I use multicity and break at LHR (IAH-LHR, LHR-MAN, MAN-IAH), it returns the message: Continental or its partners offers service between London, England (LHR - Heathrow) and Manchester, England (MAN) on Mondays, Tuesdays, Wednesdays, Thursdays, Fridays. Please modify your search criteria.
Which is fine since my LHR-MAN is on a Friday, but doesn't show me any results (just another, compact search form).

If I use multicity and break at DUS (IAH-DUS, DUS-MAN, MAN-IAH), it returns the message: Continental or its partners serve these cities but do not publish schedules or fares between them. It may be possible to construct an itinerary using two separate fares by calling 1-800-231-0856.
Which I believe is incorrect, since ITA Software says (and a manual reading confirms) the WHW5NCE fare rules allow my desired trip.

If I use .bomb I don't get my preferred routing on a simple roundtrip search, but it has no problem spitting it out when I search multi-city (albeit with UA codeshare flight numbers, prohibiting miles+copay upgrades).

How do I get my desired routing and fare on CO.com? I could pay the $10 to hold a similar trip with FareLock, but it's unclear if I'd then be able to call CS to modify the res to include the *A segs I want.
I understand UACO is going with the CO e-commerce system going forward, so I need to learn how to get results out of it.