Q/ Is the thinking that, when a stamp is affixed to a cover, the adhesive creates a relatively impermeable barrier and, therefor, the ink cannot easily flow deeply into the stamp?
I don't think whether the stamp was affixed to the envelope had an effect on ink penetration. The inks used in the 19th century didn't usually soak into the stamp, depending on the make up of the ink. The inks used for faking in the 20th century were thinner and usually soak into the stamp.
The grills were tested to help the ink penetrate the stamp to prevent reuse. It looks like that happened with this stamp on the grilled area but not the area without the grill.
I collect fancy cancels and have examples of the ink soaking through from attempts at faking the cancel. This is a good indicator of fake cancels.There are exceptions to this.