You're right - GS (Guest Services) is not going to give out cards when profiles A & B have no ticket entitlement.
Presumably, Disney will still be mailing out cards this year. (In the future, this will stop - since you'll be expected to enter special events with a linked MagicBand, linked parkhopper, AP - or eventually, your phone.)
So you buy the party tickets for A & B and have the cards mailed to you. Then you reassign the party tickets to resort guest phantoms F & G and prebook their FP for the party day at 60 days out. (We're assuming that, like last year, the umbrella function described in the first post will not function for parties - and the party guest must be on the resort reservation to book at 60 days.)
You would enter the party with the MagicBands you'll be receiving for F & G since those profiles now have the entitlements. A & B's de-ticketed cards will not work for party admission, but would work for SDFP on any day. And of course, you can book a separate set of SDFP on F & G's bands on any day other than the party day (since you'll have prebooked FP on that one day).
Another way to do this might be to reassign from F & G to A & B just before visiting GS. Ask for cards (which you'll be given, since A & B will then have the entitlement). Then, cards in hand for A & B, immediately reassign back to F & G before their FP fall off and enter the party with their MBs. (The falling off process usually takes more than 24 hours, but personally I wouldn't risk it and would stick with the first scenario.)
This is better than a crossword puzzle for mental gymnastics, right?