Well, having an OLED display connected would be tremendously helpful to see what’s going on. You can also turn on serial debugging and that way you can watch the output from serial. This would require you to compile the firmware. To turn on serial output, just uncomment this line:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/defines.h#L27
and compile the firmware.
The latest firmware no longer requires an RTC — if an RTC doesn’t exist, it will fall back on NTP syncing. I don’t think it requires PCF8574 either: without PCF8574 it will by default assume this is an AC-powered opensprinkler.