The fact that you are expressing this with such certainty makes it look like you came to your conclusion first and then went about finding a way for the facts to back it up rather than looking at the facts and seeing what conclusions could be drawn.
1. We don't really know what was going on behind the scenes. To imply that the only reason that the HD DVD add-on existed was to sabotage BluRay seems pretty narrow-minded to me.
I never saw the reason why Microsoft would add the HD-DVD for any other reason than trying to mess things up for Sony. They invested heavily in the VC-1 codec which the Blu-Ray camp played nicely by just adding it to their spec. I think Microsoft had other plans with VC-1 until it got included into BR. Thankfully it will not be used alot with HD-DVD gone.
They never had the the right hardware to actual support HD-DVD but still made a addon that to HTF could only be considered halfhearted (no HDMI and when they added HDMI it was not to full spec).
They clearly backed HD-DVD and aparently was a factor when Toshiba started to get shaky when they found out what a launch before Blu-Ray would cost. Toshibas first HD-DVD player was a incredible expensive machine to produce (it was a complete PC with a few expensive needed chips for the HD-DVD part)
To HD-DVD´s credit, the launch was what made this a fight and not a simple victory for Blu-Ray. Blu-Ray was way behind from the start and didn´t have either Hardware nor Software that could compete with the HD-DVD launch.
Until the flawless HD-DVD launch Blu-Ray was a clear winner on paper with broader support and the PS3, everyone thought it would only be a matter of time until HD-DVD would die.
What i think would be the real intersting part is some good suggestions about why Microsoft wanted to give their consumers a HD-DVD choice so early and why only a HD-DVD choice. They couldn´t expect to make any money from it, and they never thought people buy consoles for HiDef playback since they made a great deal out of how the PS3 wouldn´t benefit from BR.
Even Warner that had a part (afaik) in the HD-DVD patent pool could see what would benefit the movie industry the most.
I really hope that Microsoft makes the addon, creates a build in HDMI loopthrough and get the sound support right.
Everyone should access to HiDef