Sorry, don't have time today to read this whole thread. But I can offer some quick thoughts.
1. The sprinkler valve pictured in the previous post is not even rated for a second valve downstream of it, let alone a complete pressurized pool filtering system. So I wouldn't emulate that, if that's being considered.
2. Based on what we all just learned about BFP valves in another thread, and how they equalize water pressure on each side of themselves by venting water, even if you did have a proper pressure-rated BFP valve, the constantly changing pressure on the pool plumbing side vs the house side might be quite the water works spitting out of it. See here:
The city requires a back flow preventer be hooked up to my autofill line. The pool builder installed one. The problem is that, fairly often, it starts to vent water from the CITY side. I know it's from the city, because I tasted it (not salty at all) and also my pool water level was well...
www.troublefreepool.com
I don't know enough about BFP valves to recommend feeding one into either the suction side or the pressure side of a pool filtering system, if that's what you're attempting.
That said, there are systems designed for that purpose, that can be used "after market" when there is no dedicated line running to the pool. Pentair makes one, there may be other brands. I can't seem to find it on their site just now, but I'll try later. I'm not talking about their float valve systems, this was one you could install at the pad and it would maintain the pool's water level somehow, but it fed water into the existing filtering plumbing, not into a dedicated fill line.