1: Is this still true (firmware only supports the first sensor SN1 currently)?
With the current firmware (2.1.8) yes, but there is already a pull request in github to enable the second sensor, so we will look into that soon and should be able to get SN2 enabled soon.
2: I am limited on my in ground wires, can I use the common for one of the wires?
NO, you should NOT use common wire — common (COM) and GND are different on OpenSprinkler. If you have a switch-type sensor, it should be connected between SN1 and GND. Do NOT plug it into COM.
3: If I can use the common for one wire what would be best NC/NO. NC would seem best, with NO you wouldn’t get a failure notice if the wire or switch failed.
As I said, you cannot use common wire for sensor.
I just installed my OS.3.0 today. Worked perfect the first time!
4: Looking at your 16 Zone expander- is it similar in size to the OS.3.0? Limited space in my control box.
Zone expander is about the same size as main controller, but shorter in depth. You can find out the size by going to the product page, and click the ‘Additional Information’ tab.
https://opensprinkler.com/product/zone-expander/