Hello Fly5Guy,
I by no means am as knowledgeable as the many members on this forum and many of those that have already replied, but not discounting what they have suggested and it appears that you have utilized their suggestions, IMHO you are correct that it is screwed up and was manufactured that way. Yes as one of the others stated a P-Poor job.
The others have given all the correct answers but I think this problem is perfectly suited for your skill set. You said that you are a carpenter, so pretty good with eyeballing some thing to be out of square, out of plum, out of level. And you are using tools to help substantiate that claim. I used to work with a guy who stated that the human eye is very accurate at evaluating the three things I mentioned above, and with your experience you're probably very good at it.
It took me a little time to see it but using your second picture up against the garage door I can clearly see that the black QA plate is not welded correctly to the plow blade. Looks like an example of poor quality control.
You have already taken the others suggestions and done some more testing on your own, so maybe find a flat piece of cardboard from a refrigerator box and lay the blade face down and then using your skill set with level, carpenter square or as straight a piece of wood you can find and draw this on the cardboard to help make visualizing this easier. Then use the square to to demonstrate how out of square it is. I like the idea of trying it on a tractor in the showroom, in front of them, hoping that theirs was assembled good.
Maybe take a couple other pictures and then go back to the dealer, before it gets scratched and or shows usage, with the pictures and your receipt and politely get your point across and hopefully leave with a full refund or a properly made implement that you thought you bought the first time. But not this one.
I agree it is not correct and again your skill set and eye will hopefully convince them.
Good luck, sorry this was so long, and i will hold my breath beginning now to hear of the results. mmmmmmmmmmmmmmmmmmmm Ok i'm gonna pass out now. mmmmmmmmmmmmm