Going back to what I wrote in Post No.18, the problem is that Gateway was never designed to be part of a bigger system – one incorporating the Mk2 PV Router. Neither was the Mk2. What you have are two control systems each forming what we call a ‘closed loop’ - meaning the system monitors the result of its actions and modifies that action accordingly. What you (@ben.jones12) have --and you’ve realised this – is two control loops that are intertwined.
My thinking is turning to asking what information do you have or can find out by inspection about the internals of the Gateway? Is it possible to reroute cables and locate the Mk2 c.t. inside, maybe adding or subtracting currents by passing wires in different directions through the c.t. so that it sees a current that will give it the information it needs. Can you do what @dmajwool did in 2019?
You also mention that the Mk2 responds much quicker than the Gateway (as it’s designed to do), so would holding off its response in order to give the Gateway time to pick up or dump its load (to or from the battery) solve the problem – but at the cost of a few seconds of imported or exported power?