Bestmark is a good company in general. Reports are tedious and can be repetitive. I don't mind doing the oil change shops because I do the report on my laptop while I'm there (well, not the actual report, since I'm in the open - I do it in a Word file and then go back later and cut and paste).
You don't say what the 17 shops were. If service, I can see why you couldn't sign up. You're taking your car to a dealership, which is entered into the brand's database. They can tell if and when your car was serviced at another dealership. Since these shops are usually for oil changes and tire rotations, you can only do oil changes on each car 4 times a year (or every 3000 miles). If you go back too often it looks suspicious to the dealer.
Plus, Bestmark would probably restrict you from going back right away for a tire rotation, as that would probably mark you as a shopper. You have to act like a normal customer, who would probably get a tire rotation at the same time as an oil change (though as a shopper you can only do one service).
Same thing for auto sales. If all the shops listed were sales shops you can probably only do each location every so often. They put your name in a database, so they would know you're a shopper if you come back frequently.
The reason why those other car makes pop up on your list is they are hard shops to fill. Not many mystery shoppers own late model Caddys, lol! But you are allowed to borrow a car for a shop, so Bestmark lists them even if the car is not in your profile.
I would give them a call during the week. They have an Auto Study team, so just tell the person who answers that you are interested in doing a car shop in your state. In your case I think talking with them directly would be better than all the back-and-forth emails.
NOTE: I'm not on the forum every day. If someone comments on my post, I might not reply right away. I've been a shopper since 1991. I've never done any work for a MS company in any other capacity.