Well, you have a good grasp of the intricacies of this. IBM wanted the Digital Research code, and got it by having Gates find some guy who basically had copied it and "improved" it. But whether or not the original code came from IBM, or Digital Research, or some guy living in a townhouse in Seattle isn't really the issue. The idea is that Gates built on that foundation, just as Jobs built on the foundation created by Xerox. I don't even see that there is a dispute about any of this - even the Apple people you quoted give due acknowledgement to Xerox.
Of course they do. As do I. The point is that you started this discussion with the absurd statement:
"Then the nonsense about Microsoft stealing from Apple - when it was Apple that stole all its key ideas from Xerox in the first place."
Do you admit it was false? Especially that "all its key ideas" part? The facts are that there are significant differences and significant innovation in what the Macintosh team created.