So for those who ( quite sensibly) argued above that I was splitting a return ticket when I book a classic one way and cash return i now say bunkum! ( with a smile please)
Yes, I know it really seems that way - but you'd have to examine the fare rules to really get to the bottom of it. Lots of things might be changing in the background, that can't be easily seen from the simplified web front end presented to purchasers - the fare class might be changing, or, more probably, the fare is changing to align with fare rules.
matrix.igasoftware.com presents a lot of really technical information on fare classes used, rules in place, etc, etc. Expertflyer.com does too (though its subscription based).
I exactly get what you are saying and the frustration its generating - I have previously tied myself into knots trying to work out why its going to cost me $1000+ to fly PER-SIN-PER when SIN-PER-SIN can be as low as $400 (or even less occasionally!) on the same metal. Any amount of trying to manipulate things has failed (for me) because ultimately the 'loose ends' are tied up with arbitrary looking fare rules.
On the 'present credit card 2 days after booking' thing - I think I've hit this before for flights out of BKK ... can't remember a specific now as time has passed but I have the impression that using an online aggregator like expedia got me around that problem - don't quote me, but I think this is how I did it.
My first thought was that you have hit a web engine error - the test for this, unless its a system wide error would be to get a telephone based quote - if its the same price +- a bit then its probably real despite how ridiculous it appears. If it were me, and given you have a lot of good info now on price for booking single legs, I'd book as two separate legs, one out, one in, and try to find a method to get around the ridiculous present CC in 2 days problem (like using a travel booking site).... if that didn't work then I'd almost certainly just use a different airline to return ... I've done that before when things simply became too hard.