As Joan said, you really need to address your Abbie’s nervousness in the car as a separate issue (ie, separate from the issue of training your dogs to handle some limited “alone time.”) The most important part of working on Abbie’s car nervousness is that you have to do it slowly and gradually.
Here’s a good article on the subject:
https://www.akc.org/expert-advice/tr...-dog-ride-car/
Of course, if Abbie’s aversion to car rides is due to motion sickness, training will only go so far in solving the problem. In that case, you might still decide it’s better for her to skip “pleasure outings” that aren’t a pleasure for her. I have one dog, who is almost fifteen years old, who
always gets motion sickness in the car; she rarely even makes it a full mile without getting motion sickness, even though car rides have never made her particularly nervous. So, in her case, I generally don’t take her on “fun” outings, since vomiting isn’t really fun for her, and being with a vomit covered dog isn’t fun for anyone. Her fun outings are usually limited to our own yard or walking distance.