My final bit of evidence (for now!) is subtle.
Well, that was then. Now, it's... now, again. Anyway, here's my next piece of evidence to support my theory. Have a look at this Dig Dug re-release out of my collection, copyright 1986, Atari Corp:
Before we get back to my theory about Atari Corp not having the original cartridge label designs at their disposal... holy smokes, look at that text up top! VIDEO in one font (the original Atari Inc font), GAME CARTRIDGE and the rest in another? It couldn't be more Atari Corp! And on top of that, the sideways "5200" in the upper right logo? It looks completely wrong, and nothing like any other Atari 5200 release I've seen -- it's almost as if it was cut and pasted in randomly. Even that ATARI text itself is very slightly different -- if nothing else, it's a bit smaller. Take a look at an older Atari Inc cart that I scanned to see the difference:
On Joust, the 5200 logo looks like the 5200 logo should, and like it always does. But on the '86 Dig Dug cartridge? Not so much. I scanned both carts at the same resolution, and I cropped identical areas to compare:
The Dig Dug '86 label features the same outside-the-borders artwork that appears on the old box and manual design, just like the oddball Vanguard cartridge, and just like some Atari 2600 re-release titles I mentioned on page 1 of this thread. With Dig Dug, the rectangular "artwork area" (i.e. the art minus the title and surrounding blue field) left space for the title to fit on the label, whereas Vanguard's "artwork area" is square -- again, probably the reason they couldn't squeeze the title on there.
To my knowledge, none of the other 1986 re-releases have differences like this, and that's where my theory comes up short. One would expect to see something similar happen with Moon Patrol and Ms. Pac-Man, but they seem to have gotten those right. In a way, though, that makes these strange cartridges even stranger. Why was Atari Corp able to get most of their labels looking like the original Atari Inc labels, but not these few?
On that note... if anyone ever spots Mario Bros. or Space Invaders for the 5200 with a 1986 copyright on the game label, please let me know! Those are the two remaining titles where, although Atari Corp boxes exist for them, every time I've seen the boxes they've had the regular old Atari Inc. cartridges inside.