Sounds like diff chatter. I would flush the diff with Dexron LS fluid and see if it gets better. If the noise is still there and its been confirmed that its the diff you may need new diff clutches, I've seen quite a few broken wave washers for the clutch packs.
Idle relearn is only really required for heavily carboned up throttle bodies on c6's. Of all the throttle body cleanings that I've done, I've never had a c6 with high enough mileage to require it. If you have a high idle after the cleaning it's a simple reset procedure with a scanner.
LG make some nice headers, I've also installed quite a few kooks headers and they always have great fit and finish. Kooks headers with a Corsa exhaust has a wicked wail to it.
Mass should be able to repair it, they used to be the GM authorized repair facility, we now use York Electronics. Both companies should be able to fix it for you.
Seized plugs is mainly a Ford problem, I've never had a problem getting a plug out of a LS motor (even on plugs with 200k on them). I agree about using oem plugs on c6's, seen aftermarket plugs cause charging system problems a few times.