[asylumdown]

This amount of flexibility makes me mad jelly.

I literally just spent the last 15 minutes following all the pipes in your tank/plumbing diagram. My only advice would be:

Find some way of labelling all those gate valves in a really in your face manner. By my count you've got to change the position of gate valves on different floors 12 times in a specific sequence to complete the entire circuit from water change, to salt water storage refill, to ATO refill. It's probably the weakest point in your system as it introduces the potential for absent-minded human error. In my system the same pump that empties my water change chamber also drives my GFO reactor. Since the GFO reactor is always running, I put two gate valves between the pump and the drain just to be safe. However, twice now I've gotten interrupted or distracted during a water change and opened the valve to my GFO reactor, turned the pump back on, and walked away without having closed the gate valves to the drain. Thankfully my ATO sounds like a bucket of water pouring in to a toilet so I've caught it really quickly both times, but I'm definitely the weakest link in the system I built.

In your case all it would take is absent-mindedly leaving that gate valve to the upstairs open when you refill the salt water storage bin to either empty half your RO container on to your upstairs floor, or in to your system depending on how you left the valves after the water change.

I know it adds cost and it's redundancy you'll probably never need, but if it were me, for peace of mind I'd want there to be one pump and a dedicated run of pipe just to bring salt water up to the water change tank, and another pump and run of pipe for the fresh water. That way you'd only need to worry about changing the position of two valves (Fill ATO reservoir or salt water reservoir), and they'd both be on the same floor and right in front of you when you needed to change the plumbing 'state'. That would also make it physically impossible to accidentally foul your stored fresh water with salt water.

I'd be constantly second guessing myself and running to the basement to make sure I wasn't filling my ATO reservoir with salt water the way it's set up. But then I'm an air head. Other people might not have the same challenges I cope with on a daily basis.

However, your system saves the need for a second pump and is a way more efficient use of pipe space. I'm still mad jelly, just make sure you're extra careful.