Sorry you had an unsatisfactory experience with Meridian and Quiet Nights, but I am intrigued that because a couple tins of tobacco didn't dry out, it can be concluded that the tobacco must be "heavily loaded" with PG. That's quite a leap, if you ask me, which you could have done. And I would have answered. I suspect, actually, that the tobacco may have been packaged a little over-moist, and that what you experienced is actually a testament to the quality of the plastic overcaps we're getting.
We switched to the flat tins for a variety of reasons, one of which was the fact that the plastic caps for the older, tall tins were dreadful due to sloppy manufacturing tolerances. Apparently, the "standard" used by the tin company, and that used by the plastic overcap company did not seem to jive. To me, 2-11/16" actually means something; the two companies seem to have used different rulers. Sometimes they fit well, sometimes they didn't. I always recommended putting a double layer of foil over the tin before putting the caps back on. The current flat tins and overcaps, on the other hand, are much more accurately made, and the caps fit very well. It surprises me a little that your open tins would remain fresh for that long, but I routinely keep open tins for several months without loss of moisture. Those in dryer climates probably have different results.
Here's an experiment that demonstrates how well these caps work. Put the cap on an open tin, and press down on the center of it. Notice that it doesn't leak air. Then, lift the edge of the cap whilst putting pressure on the top, listening for the "whoosh" of air rushing out, then release the edge. Notice that the cap will remain depressed, indicating a slight vacuum in the tin. See how long it takes for the cap to return to a flat state. Or, put a couple drops of water in an empty tin, and cap it. My guess is that it will be a very long time before it evaporates.
As for PG, I do NOT use it in any of my formulae, and work hard to source leaf that has not been previously treated with the stuff, even the black cavendish that's in some blends. Even my two somewhat "aromatic" blends, Haddo's Delight and Barbary Coast, are naturally flavored without added humectants, PG or otherwise.