I checked the source code and I am pretty sure the /cm API observes sequential property of each zone:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/server.cpp#L1629
as you can see, it calls the “schedule_all_stations” function, which is here:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/main.cpp#L1047
and it definitely distinguishes between sequential and parallel. Are you sure your zones are all sequential? In any case, you are right that manually trigger a program run is now also part of the API.