2009 Tiguan dealer cleaned intake port on head now have blown motor

Took my 2009 Tiguan with 79,085 miles in for a check engine light on for P2015 Intake valve error that was under warranty.  The technician completed the warranty work and advised that we should clean the intake port due to  Carbon build up . I approved the work to be completed.  Technician “performed intake valve/port de-carb before installing new manifold”.  Within week the Tiguan started to hesitate and lose power.  
I had the vehicle towed back to Chapman VW for inspection.  Service Advisor told me “there was a scratch on the top of the #1 piston, no pressure in the cylinder and the o’ring was ruined.  He advised  I’d have to pay $1K to have the engine torn down for inspection to determine if the root cause was the result of their work or other causes.
After speaking to Service Manager, they agreed to tear the engine down for inspection at no cost to me, then advised that the root cause of the failure was due to a broken spring valve /metal fatigue and that my only option was to purchase a new motor for $7.5k.  Service manager stated ""When we perform a carbon clean, the valves are closed, we use a solution and small hand tools, like dental tools. Once the cleaning is complete, we completely assemble, run the engine to ensure good working order, and scan for codes. If everything is in working order, we deliver the vehicle. I invite you to come and look at the disassembled components to get a better idea of the scenario"


I'm not convinced that the technician did not contribute to ruining the engine while attempting to clean the carbon. Hard to imagine a car with 79K miles would have a blown engine from broken valve spring with metal fatigue. Asked for Metallurgical Failure Analysis report and the name of the lab or person that tested the spring to determine metal fatigue?  No testing done, but got this response "The black center of the valve spring was an observation that was made when disassembling. Based on experience, this was our hypothesis, not our final verdict."
I'm also trying to get info on the valve spring manufacture, to reach out to them about the metal fatigue.

Not sure what else to do.  Appreciate any advice.  This is very devastating.