And my solid advice was based on gpeso8's solid advice, which gpeso8 has supplemented with yet more solid advice.

The only thing I'd add is to keep trying. Based on my limited experience, it seems that otherwise very competent SQ CSRs have trouble with this kind of walking and chewing gum at the same time - i.e., combining J and F legs while booking with the 15 percent discount. I have no idea why you have to wait until next week for management approval, except maybe the relevant managers are off over the weekend...??? In my experience, I was put on hold for extended periods or the CSRs called back in an hour or two, but I did not have to wait nearly as long as you seem to.

I'd try yet another CSR (again, in SIN, during office hours there) to see whether s/he can simply do this now or might be able to get a supervisor to approve it while you're on the phone...or, at worst, by calling you back fairly quickly rather than you're needing to wait until next week.

I know how frustrating this is - it certainly was for me - but based on my own experience I remain cautiously confident that it will work out in the end.