I agree with basically all of this. I'm mostly talking about what might heve been going through the corporate minds of Apple and MS many years ago. I have to make two corrections:

AFAIK, there was no binary compatibility between x86-NT and Alpha-NT. In fact, IIRC, the only reason Alpha support existed at all was because of a lawsuit DEC brought against MS (but that could be a corrupted memory).

IE is not the best browser for MacOSX. I would have to give that to OmniWeb. It's a little slower, but it kicks some major ass. You should try out the 4.1 beta if you haven't already.