Right.

and no for the second scenario, until online check-in 48 hours before departure

(or, if you happened to have status on CX, even the lower tiers, both scenarios would be allowed, but I'm assuming not)

Keep in mind that this is all just the official policy (and therefore what you'd get if you solely messed around on the CX website). I don't know if a sympathetic call center agent would be willing to unofficially help you out here - I had some luck with a sort-of-but-not-really similar situation before, but a sample size of one is too small to confidently make a general statement. Maybe you could call and ask even before you make your booking?

Post #3 on the other thread you posted in (https://www.flyertalk.com/forum/cath...ave-fares.html) seems to imply that the CX system tends to prefer having your partner next to you if on the same PNR, but once again probably not a guarantee