IB, what solenoid did you use for the water? I like and dislike your solution. The like is obvious, but I dislike it also because it complicates a simple setup. I would also be concerned if the solenoid failed or got plugged and shut down all the pump (via the low water switch).
It may complicate a simple set-up, but the fact is, the simple set-up doesn't work! Believe me, I've tried virtually every new, latest, greatest valve that has come out in my 43 years associated with this business. The fact is, the float valve was ALWAYS the most problematic part of the wash. With the exception of the 571 Hydrominder, they start leaking in a short amount of time. The 571 would eventually fail to open, which was worse.
Regarding the solenoid failing to open, which I haven't experienced yet. I plumbed in a bypass that would get me up and running with the turn of a ball valve if needed. At most of my washes, my Mark VII pump stands have both a hot and cold tank. I hooked them together at the bottom and left the float valve (without solenoid) in the cold tank, but set the level fairly low, compared to the hot tank. This way, the cold valve is shut off tightly unless the hot tank gets very low. If the solenoid valve fails to open or the hot tank gets low for whatever reason the cold valve will open and function as a lifeboat.
I used the piston type Dema, 418P I think. My experience with diaphragm valves has not been good (leaked or failed "full open" when new) and I took them off and sent them back.