If I were you, I will go to a BODY SHOP store, instead of a garage.
If you go to your dealer, dealer usually won't do BODY work! Dealers sell cars, and have a team of MECHANICS to provide maintenance ($$ to dealers).
What the dealer will do is replace a brand new fender for you, which means 3000+
Also, I doubt your dealer really has spent "十几小时" to find out the problem. And labour cost per hour (plus tax) is close to 100 CAD. And that is how they make money.
Also, I doubt you can win if you sue the garage. And the problem you mentioned "色差,喷得又不均匀,还有流下来的痕迹。最惨的是,车的系统还出现问题了。发动机灯,abs系统灯,手刹灯,trac系统灯时不时就亮了一开始还以为车出问题了", you should tell the garage while your car was still sitting inside the garage. You PAID and drove your car away, that means you agreed there is no problem.
But anyway, good luck.