1a) I would think the valve still needs some minimum PSI to shut off water properly. Though I don’t know how it compares to non-latching valves. This can be found on the valve maker’s website.
1b) not really any downsides. The most common latching valves you can find on the market use garden hose thread (GHT) and non-latching ones use national pipe thread (NPT). So latching valves are often used with garden hoses, and non-latching ones are often used with underground sprinkler system. However, Orbit and Hunter (and other companies too) make both latching and non-latching solenoids that can fit on the same valve assembly, so it doesn’t make a big difference. Also, the pricing is similar for both too.
2) To trigger a 120VAC water pump, the best way is probably to use a relay (standard relay or solid state relay) and put OSBee in non-latching mode. You may also be able to find non-latching relays and in that case, you can keep it in the default latching mode.
3) To have OSBee support more zones depends on how big of the market there is. Most users still have the standard 24VAC sprinkler system, and it’s not worth the cost to replace all solenoids to use the non-latching types. So I would think 24VAC is still more common. The reason the current OSBee only supports 3 zones is that the WiFi chip it uses (ESP8266) has a small number of GPIO pins, and 3 zones has exhausted all available GPIO pins, so the only way to add more zones is to add a GPIO expander. But OSBee is designed to mainly serve as a compact controller for small-scale watering projects with up to 3 zones, and I didn’t intend for it to replace the OS.
4) The boost capacitor charges quite fast, generally 100 to 200 milliseconds.
5) Latching valves made by most sprinkler companies should work find. We’ve tested Orbit, Hunter and a couple of other no-name brands. Latching solenoids are fairly tolerance to voltage variations. Unless if the solenoid requires very high impulse voltage (like above 30 volt, which I’ve not seen so far), it should work find with OSBee.