I have not tried this particular valve, but I am told this one opens at 6VDC, but just barely. So, We know it will open @ 12VDC, and I'll bet it would HOLD open at 5VDC. If your ckt can have a voltage multiplier and give a pulse of 12VDC to open, then you can HOLD open with 5VDC.
Probably not. Adafruit has tested it down to 6v. Keep in mind the solenoid is designed for 12v. Check the Adafruit specifications here: https://www.adafruit.com/products/997