I agree that that seems like the most likely and "cheapest" way to do it, and that's what I've been saying since the rumors started - it gives Mustang more scale and more Flat Rock product.   But we can still speculate on what would better "future proof" the plan by creating/using a single platform for 5+ vehicles.  Plus the Mustang CUV also throws a wrench into that idea (though it is plausible the 4-door just uses S650, while the CUV uses CD6).   I still think that the CE1 model is something different, or I guess it too could be branded as Mustang, but I fear some name dilution if you have 87 Mustang ____ products

4 door Mustang exists for one reason only: to keep production volume at Flat Rock above some kind of breakeven point that only Ford's accountants know. Farley's interest here is to keep the 2 door Mustang alive for heritage and marketing reasons (it's one of his 4 core family of nameplates: F-Series, Transit, Bronco, Mustang) and the 4 door will add enough volume to keep building the car at Flat Rock. So 4 door Mustang will be ICE (although likely hybrid) and related to S650 or some kind of evolution of S650. There is no other business case for it otherwise.    A UEV sedan is plenty plausible as well (and Ford may very well do it) but it doesn't do anything to save Flat Rock and the 2 door Mustang. The whole point is to get more volume out of Flat Rock (which can only make sedans not CUV) and get more mileage out of S650. 

2026 U.S. Electric Vehicle Experience (EVX) Ownership Study | JD Power    

I can see the logic in that, especially if they’re serious about tying racing tech back to road cars. The question is whether Ford wants another ultra-low volume halo like GTD, or something that actually builds broader brand value. Either way, a hybrid V8 Ford GT tied directly to a Le Mans effort would be one heck of a statement.

while it might not be a direct descendant of the DEW98, I'm sure the engineering that was done for it, influenced what they did with the S197   https://majorworld.com/why-the-2005-mustang-used-the-d2c-platform-instead-of-dew98/    

I'm not talking about visibility.  I'm talking about seating position.  In Mustang, you don't sit so far down you're basically on the road like other coupes, which I actually liked, as it also meant I didn't have to slam on the brakes for every pebble.  Your proposal of lowering the roofline (and belt line) would require lowering the seating position to maintain current headroom, etc.

Urban myth.  It only shared the floor pan and fuel tank.

Ford Blue $3B Ford Pro $6.8B   Without Model E that's $9.8B profit.  Not as good as Toyota but pretty healthy all things considered.   End of Discussion.  Back on topic.  Sorry for the tangent.

A lower beltline generally makes it easier to see out. If they only dropped the roofline it would hurt visibility, but lowering the beltline as well should help outward visibility remain about the same. 

How?  That would also then lower the seating position.