I just saw the image and to me, that CPU is dead...
You have to remember that on that silicon is billions (yes...billions) of transistors/logic gates etc..so one scratch on an exposed die and you will wipe out 1,000s of transistors...which is something very likely to have happened (a scratch) when too much pressure was applied on the heatsink (as kleox54 suggested).
As for the whole (whodunnit) argument, it's your word against theirs.
But if you are to accuse a company of having botched a repair and cracked the IHS, the owness is upon you to prove that it was damaged due to their negligence or simple incompetency (something impossible to prove definitively).
The fact there there is some doubt that it could have been like this before it was taken (not that I believe it to be the case, but what the repair company will most certainly argue), remains to be determined, in small claims court, in front of a judge.
This is a longshot but you can try sending the images and/or contacting this guy (Rossman's Repair Group) by sending him an email or something, he should be able to confirm instantly if there is any chance in a repair or not.
Now, if it were me, I would write to the repair company, 1-2-3 times, with each time giving the 7 days to resolve the matter.
On the 3rd letter I would inform them of my intent to take the matter to the small claims court. Then I would take legal action if I genuinely believed that this was damaged through no fault of my own.
I would then show the photos/evidence/letters to show the courts I gave the repair company a reasonable amount of time to resolve the dispute amicably, and see what they say.
If you're based in the UK then this should be an easy fairly straight-forward process.
Altenratively, if based else-where, you'd have to consult your local consumer rights/laws for that kind of thing.