A inert round has no purpose. Same as a limp... well, you get the idea.
Exactly.
It takes a lot more heat, sustained heat, than tobacco ashes to set off a primer. I've thrown live rounds into a burn barrel to test this, and it takes a surprisingly long time for the round to cook off. Even then, I don't know it is the primer or the powder going off. Also, an unconstrained round, a round that isn't chambered, poses very little danger. Projectile velocity is damned near nil (the projectile has to be in a barrel, gasses trapped and expanding behind it as powder continues to burn the length of the tube), although you might get a nasty powder burn if you were holding it.
I've made up a bunch of inert rounds for testing cycling, etc, but honestly using them as tampers would just take all the excitement out of it.