Who just emailed with a link to this thread? There is more than one issue here and more that on person.
The z axis being out of square is simply an imbalance of tension between the top bolts and the bottom bolts. It is much easier to fix that than the x/y being out of square.
1mm over 83mm is .69 degrees out of square. I hope you agree that is very close, and I assure you if you need it closer a little bit of very careful adjustment can get you closer. I would also look into how perfect the bed is, you are getting into territory that is hard to measure. It could be many things.
I have this thread and your emails, and this thread has two people in it other than me, this is very confusing. It is difficult to compile the two into your issue. You asked for new parts in the email. I feel .6 degrees is so close that it can be adjusted a little closer but honestly I am not sure what the problem might be or how “perfect” any printer can get. The parts are plastic and they do flex a lot, and are designed to so they can be adjusted. If you look hard enough every printed part isn’t perfect.