the car selection (in the past) has been more than just marketing, except for the obvious fact that they stick to Nissan cars. They also seem to have something specific to teach drivers in each academy round, and the car fits with what they wish to teach.
The tests seem to be a series of "well, can you drive like this? And this, and this? Okay, maybe we'd be interested in putting you in a real car..."
This is part of the reason we see shittier tires and car setups that don't "jive" very well with some players. It's a test of what you can handle and still shave off time week after week. That's the kind of thing they are looking for in the National Finals too - to improve a bit over the week, and to have all situations thrown at the driver to test them.
To me, there is engineering of a race driver happening at every stage/round of Academy. Sure, the marketers have their say, but the race division of Nissan clearly has refined their methods because I don't think they've been entirely happy with the graduates 100% of the time. They want a better chance of having THE race driver of the future emerge from the program, and not just flash the Nissan symbol on everything. Though, both are complimentary