timeline - it's your call whether or not you pursue action. I'd be stunned if you weren't allowed to pursue the whole amount, plus the $300.
However, it'll be time consuming and annoying - mine took a year of letters, phone calls, documentation, testing, etc. and then a 30 minute court date. Actually, the court appearance was the easiest part of the whole situation.
As to proving failure or otherwise, that's really not timeline's problem. A simple statement of "on such and such date, I took the car to XYZ shop for specified repair/issue. Car was returned on such and such date with XYZ shop stating it was repaired and in proper working condition. Car was not returned in proper operating condition as evidenced by symptom, behavior, etc. Subsequently, car went to ABC dealer on such and such date, ABC dealer confirmed car was running out of factory spec and proceeded to return car to said spec using parts already installed by XYZ shop. ABC dealer commented that parts were marked with identifying marks, something not to spec for this repair, etc...."
Provided the parts given to the shop were VW parts marked with proper part numbers, etc. then that should establish their veracity....
Like I said, though, the decision ultimately is timeline's. I can appreciate if he doesn't want to fight it or deal with it. Sometimes, other priorities take hold. Having said that, timeline, I took on one of the largest VW dealerships in my area and had really no trouble presenting my case to a judge.
I guess my last point would be to say that I didn't want to try and cause a huge debate; However, I did want to offer up a differing point of view given my recent experience taking a dealership/shop to court over a botched repair. I did want to point out it is possible to fight and win - there were many times during my fight where I thought I should just drop it. However, a few close attorney friends as well as some club members here kept me focused....