That's funny
Communication is difficult, and written communication is so difficult!
Anyway, as I mentioned, my setup is still very nice on Zwift. I get 3 sensors telling Zwift what I do (speed/cadence/HR). Then Zwift displays the power and my target power. And whatever the climb, 200W is 200W, only the speed changes. I change the resistance myself with my gear. The output power is just a combination of what I'm pushing (cadence) and the gear ratio. What you need to understand on that Fluid trainer is that the resistance increases with the speed of the rear wheel.
Yeah, I had a feeling the 600W was just some sort of a miscommunication.
However, to the Zwift: You are right that 200W is always 200W and only the speed is changing. That gives you speed x on a flat road. When you start to climb at the end of the flat road, your speed drops to speed y, assuming you are still holding your 200W, and y<x. Obviously, you are on a trainer, the incline does not change and the resistance of the flywheel in your case does not change either. So if I understand it correctly this time, the Zwift's "brain" knows that you are climbing and drops your displayed speed appropriately to the incline of the virtual hill. Then, if you want to maintain your perceived speed in the virtual world constant when hitting the hill, you need to upshift to a harder gear and pedal harder to increase your power output. That is the difference comparing to the real world or a smart trainer where you just need to pedal harder without the upshift part! So in conclusion, with your settings if you are recording your training session both in Zwift and, let's say, with a Garmin head unit, at the end you will get too different readings regarding the ridden distance and the speed profile.
As I said, I am a little slow but hopefully I got it right this time. So proud of myself