Mark,
Mmmmmm. Fun! Sounds like you've got some entertaining times ahead.
I reckon you're dead right about the rear bar giving you extra options to balance the car. If it rains before a session, you can drop it quick to make the car more predictable.
Well... my only thoughts would be that *IF* the car is the car is balanced in a long turn (when it's had time to settle), I probably wouldn't look at the bars and springs. It sounds like your problems are when the car isn't settled (transients), and that's where shocks really come into play.
Still, might not be suspension at all. Pro might well be on to something regarding the brakes.
I'm not sure if it's relevant, but I got told an interesting story once by a talented race engineer (8 national championships, IIRC). He said that one weekend the driver was complaining of exit oversteer. So they played with the suspension to correct it. In the next session it was worse. They played some more. Worse again. And on it went...
In the end, the driver just happened to mention that the car was now suffering from entry understeer too. It was there before, but now it was really bad.
It all clicked then. The car was understeering. The driver would chuck the car into the turn to get around it - but then the car was unsettled. When he hit the gas, it would oversteer. All of the changes they had been doing only made the root cause worse. The changed the suspension in the complete opposite direction to reduce understeer - and the oversteer went away!
This changed his mindset on setup after that. He always asked the driver what the car did on turn-in first. Everything that happens after that is just a response to turn-in. Get the turn-in right, and the rest should follow.