Would it be assur for a biglaw lawyer’s associate to come to their house, and show them some changes in a document, and then go execute based on his advice?
Yes, absolutely assur meikar hadin.
As an aside, you should really see the meforshim. You can’t take a gemara and read it against all the meforshim in order to fit your agenda. That’s a lot closer to kefirah than anything else I’ve seen on this thread.