First just change your thinking from 2 for 1 to 50% off. That is what it is and the singles get the same 50% off the standard fare and then pay the applicable single supplement. Same value for double occupancy or singles.

Onboard booking has just become complicated. $200 per person now.

If you do an "open" booking (which means you say just take my $200 and I will decide later) it is non refundable and you must book a trip within six months or lose your deposit. Even when you book a trip within the six months if you cancel that trip you still lose the $200. You can transfer that $200 to another sailing but if you dont sail eventually you LOSE that $200 pp

If you do a "real" booking selecting a ship and a date then the $200 p.p. is actually refundable. If you cancel that cruise down the line you get a refund of the $200 p.p. You can also transfer it to another sailing.

Yes, I know it is not the most easily understood policy but that is the way it is right now.