1) If you use a DC power supply, you may see the effect that the valve will turn on but will not turn off. Because triacs (which is what OpenSprinkler uses to switch solenoids) are only meant to switch AC current, not DC current. However, if you have a small load (like an LED light or something) it should still be able to turn them off. For additional details, you can read about how triacs work to see why they are not meant for DC current.
2) manual_run_program will indeed immediately stop all existing running programs. You can see that in the code of that function.