1. Fender extensions. JD has them for the 3x20 series, stands to reason they should have them for your 5083E. Right side installation dead simple, but I had to have my dealer install the left side due to fuel tank proximity.
2. Warranty, no question
3. There was minimal - if any - dealer prep work done on mine. It's almost like they just took it off the truck and handed me the keys. I suspect they didn't touch your mower either. Slip clutch definitely mal-adjusted since delivery, try the warranty route first. Your description does really sound like one of the blades is jammed, causing the unbalanced rotation. And if the PTO shaft was bent, you'd definitely see the wobble. But if one of the U-joints was damaged and loose, you can't see that without taking off the covers. Unfortunately they're not very user friendly when it comes to removal. Again, try the warranty route first.
//greg//