PE = Professional Engineer (I think this encompasses all licensed engineers, EE and structural may apply to this project) stamps depend on whether your AHJ / POCO require it.
I think what you're running into here is, installing a rather large DIY system on a service setup that is at the edge of what is common on this DIY forum, and just outside what you can find articles explaining it. For instance I self-designed and installed a 17kW-AC system on a 100A service, which allowed me to avail of some simplifications not available in your situation.
I don't trust the plans to say what the busbar rating is on the service disconnect, versus having a part number / picture of the wiring diagram on the equipment itself. The two sets of plans disagree anyway.
If the 200A lug fed subpanel is convertible (IE can accept a main breaker), then main breaker would definitely allow solar tap to go into the same 200A disconnect. And you can kind always replace that subpanel with a convertible one. 200A convertible is usually not expensive, like $100-150 with the breaker.
Looked at the photo of the top of the subpanel. I don't think a main breaker can dock in the lugs at that place.
This stuff isn't exactly rocket science, but it's not been super efficient/reliable to go back and forth on this forum. Maybe you can see if a professional is willing to VC with you? Then they can tell you things like, "move the camera 2 feet to the left so I can see the wiring diagram", "pull that thing out".