DISCLAIMER: please be advised that I am not your attorney and there is no attorney-client relationship by providing this to your question posted anonymously without facts in evidence. Please consult with as many lawyers as you wish before you make any legal decision.
I hope this helps.
As per questions number 1 and 3, instead of bringing any proper legal claim against the repair shop, I would inform your car insurance company what happended by filing a claim. If the reapir shop had a responsibility to take proper care and custody of your vehicle, and because of their negligence, your car got stolen, your insurance company should be able to resolve the matter with the shop for you. Let your insurance company worry. That is why you pay its premium. (Your insurance company will pay you present market value of your stolen car)
But notice that the repair shop may also not be liable for someone's criminal act, provided that they were not negligent in securing your car while it was on their garage. Nevertheless, most auto repair shops do carry garage keepers coverage which will cover a theft while a customer's car is in their custody, care, and control.
*in small claims court, you should represent yourself. But by following proper procedure, you may request interpreter.
As to your second question, please be noted that you have a duty to mitigate damage. If purchasing a new vehicle was a reasonable way to mitigate your damages caused by loss of use due to undue delay in its repair, then you might be able to ask for its costs. Vice versa.