So right off the bat I noticed that the thing that counts your points either for clean driving or trading paint doesn’t work. I’ll be driving through a vineyard getting destruction points but as its racking up it doesn’t count stuff after the initial hits, or for the pagani zonda bucket list it makes you do near misses to complete the challenge. The only thing is I would get really close to 5 cars in a line but it wouldn’t count any of them. Plus a championship needs you to get a new car and I decided to buy this car for 85k credits but it told me that I could get it and other cars from a pass. That means the season pass only unlocks the cars you still need to pay credits for them.
Not sure on the first part of your problem but no championships are limited to just DLC cars, you normally get a choice of several cars. Once you get the car pass though and it’s all installed the cars that are part of the monthly packs are I think all free to purchase the first time then cost in-game credits for any more you wish to buy.
Yes the purchasing the cars in game with credits can be a bit confusing when you have purchased car passes.