Hi Cliff,
Here is a new suggestion for improvement.
I think some are minor and should bring more ease and ergonomics to the interface.
1: Adding a game loading icon to not go back through the main menu - There is only one icon to save.
For the showroom screen:
2 - Add a slider or plus and minus buttons in the showroom screen to change the price of cars without having to go back through the design screen.
3 - Display in an insert the number of models present in the showroom. (See my message about filters)
4 - Add an icon on the car’s detail if a discount is made
5 - Classify the functionalities in two columns possibly three. (One for the name, one for the price and possibly one if the features is to serial or no).
The design screen:
6 - In the functionalities, it would be necessary to add something allowing to say if a equipments is of series or in options. The customer being king, it should therefore be able to add heated wingmirrors on a model equipped with electric wingmirror. 
(After a certain time or car sold, this feature becomes standard)
7 - Added a shortcut button to access the scheduler.
8 - Rework the graphical interface of the detail of the features costs
9A - Add the major design categories: (Motorization, Color, Design, Equipments…).
9B - Also list everything that makes up a standard vehicle.
The income screen
10 - Add a sorting column per amount of the car model
11 - For ease of reading issues, as in most management games, it would be nice to income writting in green and red for spending.
That’s it, I think that’s all for now. If I see anything else, I’ll give you a sign