I know similar questions have been asked before but haven't been able to work out whether what I am trying to do is worth persisting with...

Brief scenario:

Want to take family of 4 on trip to Singapore/Malaysia next October half term.
Have 2 Amex 2-4-1 vouchers.

Grabbed 4 outbound CW seats LHR to SIN as soon as released on BA.com - used 2-4-1 vouchers - cost 140k Avios

Waited watching for next 10 days for 4 CW return seats from SIN or KUL to open up.

2 days ago - 4 SIN-LRH return date seats appear at midnight - by 07.30 when BAEC lines open to add them, only 1 left...

Next day - 4 KUL-LHR seats released - I bottle it - book them online in the middle of the night for 280k Avios on separate PNR

Call BAEC when open at 7.30 - ask if I can move the return seats on to the original PNR so the 2-4-1 vouchers apply

Agent very helpful - explains risks of cancelling 2nd PNR (5% chance I lose the return seats) and then say she will email the back office to request the seats are moved (no promises). She also adds a 'ghost' sector to PNR 1 for the return seats to be moved in to (this shows as 'For information only' in Manage my Booking). She mentioned that this is what she had done with previous such requests...

I call back this morning - long queue - sounds like call centre abroad - given short shrift - not possible except by cancelling return seats and rebooking if they are re-released... I rang off.

Should I keep on trying?

The original agent appeared to indicate that seats had been moved on request before.

Has anyone else ever managed to persuade them to combine separate legs of a trip on 2 PNRs?

If this has been answered before then feel free to point me at the answer - as I say - my 'amateur' forum searching has left me undecided whether to give up on the extra 140k Avios I paid... (unless of course 4 more return seats later open up in which case I know what to do!)

Thanks

Matt