The Opel Diplomat was more advanced in some technical areas than starting from the Nova platform. De Dion rear suspension and 4 wheel disc brakes. But, it is true that production tolerances for it were tighter on the body structure than was typical for a then American GM build. The consideration for using the Diplomat platform is that GM could have used existing tooling to reduce development time and outlay.
Remember that the Diplomat's concept was to be a German competitor to Mercedes-Benz and was already engineered using the Chevy small block.
GM as well as the American Auto Industry missed where the luxury car market was going.
They wanted to stratal the market from where they were to where it was going and not fully commit to the Euro style that was coming in.
John Delorean got hot when he was at Pontiac with his work on OHC engines, Fuel Injection, Radial tires, Composite headlamps, Rear disc brakes HEI and more but he was looked at as a heretic.
So Gm takes him and puts him at Chevy over the pending Vega mess to where he was set up to fail with a program he never would have pushed for.
GM could have done more here in the first Gen Seville as they could have at least put on the TA suspension under the car and made it handle This was not a car to float down the road and that is why it has always struggled. Later GM saddled it with FWD too. It was a car GM just never knew what to do with it.
Ford was even more lost with the Lincoln Versailles. It was an even greater mess.
People in the later 70's finally discovered cars are to go around corners with some level of competence and it took them a while to make them do it with out truck springs under them. GM has gotten it right decades later understanding the shocks play as much a role as the springs and that larger tires are not the only thing for grip.
I loved the look of this car and always wanted to find one to put the Trans AM suspension with the Herb Adams VSE bars and shocks under this. Drop in a modern V8 and really see what could have been done.
The next gen really showed how much GM product planners lost their way. The Rolls like retro trunk was a major move but in the wrong way. I know some loved it but it really did nothing to advance the brand. Add in the Olds Diesel and it really was in trouble.
The down size Seville should of never been. Then finally we got a good looking car with a DOHC V8 but driven by the front wheel?
The Seville is a good example of what was wrong with GM in these years. I am a die hard GM fan but I have to call it what it was.
Imagine the black and silver special edition with a proper suspension and a better engine under the hood. With seats you sat in vs on. A manual tranny? A car that really could tour not sway.