From what you are describing there, thats exactly how secondary groups already work. you would have 3 groups
Car owners
Chevrolet Owners
Ford Owners
Member x has
Main group: Car owners
Secondary group: Chevrolet Owners
And in fact its actually more flexible, as member x may then also purchase a ford, and want to have access to those too. So would be
Main group: Car owners
Secondary group: Chevrolet Owners and Ford Owners
Now for the groups themselves when you set up, you set up car owners. Then you create 2 copies of car owners, and then edit their permissions to give access to Chevrolet owners sections and ford owners sections respectively. Reverting would not be needed, as you would just remove the secondary group
As for emailing, you can target any group you want, just as you can now.