Try use the scale /100 button, to divide the model scale by 100. Then the car should be closer to the right size.
No, it's not what I remember from official docs where Ruud states to re-dimension the car instead of rescaling it via Modeler.
To create car.ini file, take 1 car.ini file & tweak it from there. Begin with transferring values (xyz) from your car to the car.ini file for the flares, backfire, lights, wheels radius, suspension, aero...those are the most important ones.
Debug your car with racer.ini => dev.note_ini_fallback=1 & keep your QLog.txt file in NP++ opened. Check & fix problems "chronologically" & always debug to check everything ingame. Delete eventually your log & re-enter the garage, check your Qlog file again.
Multisub mats should be verified prior exporting them, so make sure they have been assigned correctly. It takes some hours of testing it & getting good results in Racer.
We know about your problems, it takes time to understand & know what Racer is about, take your time & make sure you read & learn a lot from Ruud & Mitch official docs. Here's the links :
http://www.racer.nl/reference/
http://www.racer.nl/tutorial/
Keep faith, if you want something, you can get everything you dreaming of, it's a matter of will & time !