What a thread this turned out to be!

Ultimately, all 3 bookings are made with the same T&Cs. 3c4 applies to each of them. Now, I'm in a position where I know how ambiguous this clause can be as I've had multiple GGL agents give me a completely different answer in the past - and I know other GGLs who have had exactly the same thing. So there's definitely a level of agent ambiguity on this one.

Had OP not found an amenable agent, would they have been in a position to buy a walk up fare to SIN (or SYD if timing was such they had no chance of meeting that connection either) and then issuing MCOL against BA on their return? You'd almost certainly be successful, but it's still a big step to put that kind of number on your credit card, for most folks.

Ex-EU works well for me - but is not without risk. And, as CWS has repeatedly stated, at airports where B2B is possible (such as GOT) it should always be the choice when booking these fares.