Go to another tint shop and talk to the manager. See if he'd mind having one or two of his guys (guys who actually work the cars) take a look at your car. Have them look it over (they've probably not worked a 3 before) and ask them if based on the location of the scratch(es), see if they think it could be caused by improper removal of the spoiler. A competitor of the tint shop you went with should be more than happy to comply when you mention you're sueing their competition.
If they think it caused if, have them write a formal letter, signed and witnessed as their testimony that they believe the scratch on the window was caused by the removal of the spoiler. I dunno if that'd hold up in court but it couldn't hurt.
You've also got a pretty good case as you mentioned the dealer should have checked for scratches before hand and if he saw any, he should have denoted somewhere on the paperwork you recieved afterward that "Rear window scratched upon arrival". Now he could easily forge something like that after the fact if he really wanted to, but to hold up in court you'd think you'd have to have signed it prior to the work being done.
Take his bluffing ass to court.