also if it helps cos just did this for family for a trip to Europe: you can call up to separate out the APD to be used on specific person(s) that way you can have the normal fare ticket for full SP earn (aka pay for kids with APD, and pay cash for your own ticket) and still be on the same PNR.

regarding mixed-airlines itineraries and APD = doable, above is NZ > BR, and i've also used APD to part-pay for a NZ>KL intinierary