Nicely done. Absolutely excellent first article.
If I may, I'd like to clarify a couple of points. I wouldn't classify XML as a separate plan type (although I probably did in older editions of my book). The XML plan and the graphical plan are one and the same. It's just how they're displayed.
For when estimated and actual plans differ, again, nice job, but you might have made the point a little differently. The examples you show are likely to have the plan itself be the same between estimated and actual, but with differences in the details. The main cause of differences between estimated and actual plans, when the plans change, is recompiles.
On the parallelism, that's a mistake I made in my first book. It's wrong. There are not two plans. There is one. Sorry.
Again, nice job. Looking forward to the other articles.