I plan on doing this in the future so, I follow your thread, and it seems that you worked through your issues. 1 question about that, do you have the entire sequence happen right after the ATO topped off so that you know that what you removed is what you replaced?
My line of thinking and what concern me is if the tank has evaporated (lets say 0.5 gallons of) water and before your top off was called to replace that deficit with RODI water, the apex triggers the water change event and removes about 2 gallons it will then refill to the float switch about 2.5 gallons of salt water, the extra 0.5 gallons of saltwater will displace that 0.5 gallons which will slowly raise the salinity up over time.
point being that in my mind, this is what makes or breaks a good automated water change system, if you remove exactly 256 ounces of old salt water, you want to replace exactly 256 ounces of new salt water.
I like that hopper!! one thing to point out, if you're going to use an electric solenoid for salt water applications be sure that the the materials inside the solenoid that touch salter water are reef safe, ie stainless steel or better yet titanium. If you research these types of solenoids you'll quickly realize that the price jumps dramatically when it needs to be reef safe.