The burden of proof is on you. You've already said you have access to free legal resources to help you - seems to me that you'll have a lot better luck with them as opposed to arguing on an internet forum.
Well he did say he's still under Applecare. They should have at least checked the thing. Saying there was a storm means it's likely that a circuit was blown here, but there is no guarantee of it. They should have at least examined the problem.
I had a friend who's iMac was fried when lightning hit the utility pole in front of her house. Apple gave her a new iMac. I am betting that she was very cordial, did not insist on a solution, did not argue legalities, or use words like "prove it".
Anecdotes are silly and not even remotely helpful, because they aren't necessarily repeatable. You heard from your friend that she received and exception. This does not establish any kind of precedence for anyone else. It's a different day under the discretion of a different employee. You have nothing here. You're just implying that perceived rudeness is the reason this was not repaired, and that as a matter of policy is ridiculous.
How does someone stress test a computer?
You can use something like Prime 95 run over several hours. For ram you can run several cycles of memtest in single user mode. I do that when I buy new ram. If any of it generates an error over 5 or so cycles of memtest, I send it back, as bit flipping should not be a common occurrence. It can sometimes reveal problems that may not yet be evident under normal use. It's like with a dying gpu where it still brings up your desktop, yet you may start to see artifacts within either a demanding game or application that uses OpenGL drawing.