999 times out of 1,000, "square-cornered variations" of cards (or card-like items) that were issued in rounded format are items that were cut from full sheets on the secondary market. (They fall in the same category as the items touted on ebay as "oversized proofs", "2-card panel proofs", and any "x-card panel proof".)
It looks like you're going about this the right way, to determine if these were an actual legitimate release. What would be ideal is if there is video somewhere of someone opening these differeent packs and pulling these variations. (Of course, that's also assuming the packs are legit and came from the proper source and weren't just some attempt at a "re-pack".)
There are a lot of points that will need to be fully covered before these can be listed here. Thanks for seeing to it that they are being covered.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).