Took a look at your project and I can tell you’ve been trying a lot of different options. The best thing might be to start a new collection from scratch. I suggest creating a collection called Photos or Images. This is where you will upload the images to be used in the “gallery” on each car page. Not sure why you also have a thumbnails collection. It shouldn’t be necessary.
Each photo can have additional fields as necessary. See other topic below for more in-depth explanation on how to create an image library using a collection. It also has a link to my clonable project sample.
Create a multi-reference field on the Cars Collection and link it to the new Images Collection. For each car you’ll select the related photos.
On the Car template page add a collection list elment and connect it to the multi-ref images collection.
Now you can start working on the lightbox. I recommend this trying this method.
Hope this helps get you going in the right direction. Happy designing and let me know if you have further questions. Cheers.