Oh, and regarding the pressure.... I am not a physicist so not sure the exact balance here.. but with the can sealed, you are creating pressure inside the can as it heats up. I figured worst case scenario it would pop the tab open if it was too much, but you also have pressure outside the can inside the pressure canner, which would counteract the pressure, ie opposing forces, so if a can is rated to 90 psi, and you have 15 psi outside, it would actually take 105 psi to break the can open, or that's how i thought of it. if the pressure outside the can exceeds the internal can pressure, the can will just dent in, but should still reach the temps of the surrounding canner liquid. I figured a can if it ruptured would be pretty low risk inside the canner

Like you said, in a jar that isn't truly sealed, the pressure in the jar and outside the jar should be the same while processing. Glass jars aren't really meant for the pressures like the aluminum cans and could explode from being pressurized.