Actually I don't think you can ticket a DONE4 that has HL in it.

When you want to allocate seats, you can actually use the aa.com online reservation retrieve tool and select your seats. Same for BA flights through MMB.

Unfortunately CX's CUPID does not link too well with other Oneworld partners.

As for the ticket deadline, CX's policy is 1 month prior to departure. But of course you can negotiate that (depedning on the mood of the CSO)
For DM MPO deadline are moved.

BTW, if you don't mind me asking what is the price for your DONE4 ex CMB, as I am planning my DONE4 ex BKK and I find CMB is too far and cost more $$$ to get there and the savings might not be worth it (only around 300USD?)