1. Assuming you have OpenSprinkler 3.0 — there is a bug in the current firmware 2.1.7 that basically ignores static IP for OS 3.0. This can be easily fixed. In the meantime, I highly recommend you to use DHCP reservation (as you are already doing) as that is the right way to do static IP (setting static IP on the controller itself is not recommended).
3. If you are using Zimmerman algorithm, then it directly changes the watering percentage, which you can see on the homepage of the controller. The controller also logs the watering percentage every hour. If the watering percentage is 0 (and if the program is set to Use Weather, then it will not run).