Sorry, I was reading your number 1 and 4 as making an argument for this to exist. I just reread it and you are right. I just found out this may have happened to another person. I'm waiting to see the scans.
I saw a whole bunch of sketch listings cancelled with either highlighting or color. I wonder if these may have been enhanced/altered as well. If the backs were scanned, you could tell by the copyright, but only the fronts were in the auction.
Additionally, I would call them altered, and not fake. From what you've said, they are actual sketch cards, just altered. That would indicate there's not an endless supply, but you must beware of the ones you receive, and at the very least, check the copyright about coloring.
This would be a perfect use for the sketch book/magazine talked about before. If the sketches were catalogued (great additional product), the catalogue could be used to at least verify a sketch image as genuine in addition to its aesthetic appeal. It could be available online with watermarks, and a link could be written on the back of the card. Anyone with any doubt could simply logon and check the card they have.