Will a zinc plated Steel 12mm X 1.75mm hex bolt stand up to the heat of the exhaust, or is a gulvanized bolt needed?

 

Galvanizing is just dipped vs. plated zinc.
Both will last awhile.
Stainless is best, if you're keeping the car for some time.

 

Go to mazda and buy the distortion nuts

 

distortion nuts?

 

Distortion nuts - AKA elastic stop nuts.
I take SS nuts and sqeeze them slightly out of round in the bench vise.

 

is the dealer going to charge me like 100 bux for one of them

 

Distortin nuts are not "AKA elastic stop nuts"

Nyloc: Nylon Insert nuts will not stand up to the heat of the exhaust manifold

Distortion nuts are all steel and eliptically shaped, so when they are run down the thread the nut expands slightly to grip the male threads.

3rd gen style turbo distortion nuts are ~5.00 each