Well I'm glad at least one Sheriff has seen the light lol.
It's really not such of a confusing issue, really. If you are 18 you can purchase a handgun in the state, but not from an FFL dealer. You just have to get the PPP first. Once the gun is in your possession there is no more need for the PPP. By carrying a copy around to educate LEOs, you're using it as a carry permit, which does not exist in this state other than the CHP. To me, it would only add more confusion to the mix.
PPPs are only used in the transfer of handguns and that's it. All you really need for proof is an ID card showing your age. It's up to the LEO to understand the law and act accordingly. You already did your job by getting the PPP prior to taking possession of the gun. There's no need for that Sheriff to adjust the wording on his PPPs, either. Any FFL dealer should already know about the Federal laws against selling handguns or handgun ammo to persons under the age of 21.
But anyway, you don't have to tell anyone that it's a borrowed handgun. If one is 18 and has no other legal disqualifiers, that person is legal to own and carry openly where allowed just like anyone else.