Have you tried using the F5 and F6 keys?
It's probably not the answer for you, but others might have a similar problem, and might have missed those keys. I had the problem of no backlight for the past six months with my MBP, before finally stumbling upon a forum comment about it. I'd tried to find ut what the keys do, but in bright daylight, and nothing that i had seen even mentioned theose keys.
This sounds simple, but it's true: I had another MBP for more than 5 years and never used those keys; the backlight worked, and so I didn't need to investigate further. :-\