Here is what I would consider a reasonable approach:
Start with the premise that one is required by Appendix G to have a single air loop serving the entire floor.
Set the design SAT for this airloop at the worst case requirement (15C in your example).
Allow the SAT for this airloop to reset up by 5F from the design SAT using a SPM:Warmest
Size all the zones served based on 20F supply air to room air temperature difference, as required by App. G.
If I understand correctly, this can be done by setting the Zone Cooling Design Supply Air Temperature Difference field in the Sizing:Zone object.
Control your zones in the simulation to follow the desired setpoint schedules as you described (that is, some zones are allowed to maintain higher temps in cooling mode operation.) The system will supply air at design SAT of 15C, resetting up to 17.3C if connected zone loads allow, and it will operate to meet the scheduled setpoints in all of the zones served. The zones with looser temp control requirements are unlikely to set the SAT requirement when SPM:Warmest executes.
The only potential concern that I can think of is the risk that some of the loosely controlled zones might be overcooled at the zone minimum cooling flow rate.
