1) I don’t think so — your contractor may be referring to driving the solenoids with 24V DC, but OpenSprinkler DC is specifically designed to drive the solenoid with an impulse voltage to engage it and then use a low voltage (7.5V) to provide holding current. This amount of current is compatible with the amount of current running through the solenoid under AC. So I don’t believe it shortens the span. In addition, plenty of people in the open source community built controllers that drive the solenoids with 12VDC (without the impulse-holding mechanism that OpenSprinkler DC uses), and have been doing so for a long time. So overall I think the risk of lowering the solenoid’s life span is very very small.
2) The claim that ‘AC travels better than DC’ should be taken within context. It all boils down to ‘wire resistance’. Power is wasted on the copper wires as electricity travels through it. The power loss is basically square of current times the wire resistance (I^2 * R). So under the same amount of current, the power loss is the same, regardless of whether it’s running AC or DC. As I said above, DC OpenSprinkler achieves roughly the same solenoid current as in AC mode, so the power loss is similar too.
Where ‘AC travels better than DC’ comes in is the fact that it’s easy to step the voltage up and down with AC. To transmit the same amount of power, higher voltage means less current, and less current means less power loss. So AC wins because you can use transformer to step up the voltage at the transmitter side, which reduces the current being transferred, thus reducing the power loss. On the contrary, it’s not so easy to step up and down DC voltage. If you want to know more, you can read the story of the fights between Thomas Edison and Nikola Tesla, the famous war of DC vs. AC.
3) Not sure how far your solenoid is away from the main controller. If it’s within WiFi range, you could get another opensprinkler and sets it up as a remote controller listening to the master controller. Remote zone is a standard feature supported by OpenSprinkler firmware.