As I follow all this I am sick to my stomach for the owner! I know you can't compare the 2 but it reminds me of back in 1996 My Dad and I both bought matching Ford F-350 power strokes. Mine arrived first I paid for it in full cash, drove it 18 miles and had to pull over cause the motor sounded like it was going to fall out of the truck! Long story short right away the dealer trying to get me to relax said "we will get you a new motor" I said no you will get me a new truck! ended up being all the rockers on one side were not adjusted properly and one actualy fell off and a valve stayed shut causing all my problems.
My dads truck never made it off the lot, went to check it out the day it came in looked under it and it was leaking oil from the pan! Due to the sump for the cross member the engine had to come out to remove the pan for a new gasket and reseal. So on a diffrent level I can relate to the grief of this thread. I truly hope for the best possible out come for you and you some how get even more out of GM for the major heart burn you must have from all of this!
I am due to pick my Z up today and think if this was me I would have gone postal by now
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Best of luck and keep as positive as possible