if a guy bought a brand new pistol grip shotgun, could he legally cut the barrel down to 10"?
I want to see where this is discussed in legal code (either Indiana or through batf)
Negative. If you want a 10" barreled shotgun with a pistol grip in Indiana it must leave the factory in that configuration. It is classified as a smooth bore pistol or Any Other Weapon (AOW) and requires you to obtain a $5 tax stamp to possess.
Question 2
if a guy bought a pistol grip shotgun, could he legally possess it in his vehicle without a carry permit?
Yes, provided it is unloaded, secured in a case and not readily accessible. You must meet all three criteria to transport it lawfully in a vehicle without a handgun license.