1) By default the firmware uses UTC time obtained from RPi, and the time zone data it stores, to calculate the local time. The RTC is not directly used by RPi. Instead, as long as RPi is online, it will automatically perform NTP sync to obtain the current UTC time. You can set up RPi to sync with the RTC, so that even if there is no Internet, it can still get accurate time from the RTC. Again, setting up RTC is a separate step that’s apart from the firmware.
2) I believe so. If you have multiple controllers, the OpenSprinkler UI/app supports multiple sites, and you can give each site a different name.
3) The device ID is not relevant to RPi. It’s used in the past to distinguish multiple controllers on the same network. The background is that in the past the microcontroller-based OpenSprinkler had software-defined MAC addresses, and the last byte was defined by the device ID. RPi always has hardware defined MAC address (and all current versions of OpenSprinkler have hardware defined MAC address), so the device ID is not relevant any more.