Neither Mercon V or Mercon LV will harm a 4T65E, It likely has a Dex/Merc fluid as it's usually the cheapest bulk available.
You could be smelling paint if they painted the case.....The exhaust crossover is quite close to the bellhousing. But definitely look it over for leaks!!
A TransGo SK shift kit usually cures Accumulator problems, I can't imagine a builder not installing a SK kit in a 4T65E?
The harsh "Garage Shifts" have me concerned & could very well be a PCS/EPC or Line Pressure control issue.....The only way to KNOW is with a Pressure Gauge!!
Have you contacted the shop that built the unit?