Thank you for your advice, yoavf. You write:
To achieve what you want, you'll need to keep the DNS of the domain with your registrar, and only point blog.williamalba.com with a cname to wlva.wordpress.com.
This is what I originally did. From paragraph 3 of my original post:
On my 1and1.com control panel for <williamalba.com>, after creating subdomains "blog" and "www", last night I created a CNAME alias in the <blog.williamalba.com> entry: <wlva.wordpress.com>. I also paid $13 (per year) to WordPress.com for the subdomain redirect. In other words, I followed the directions for subdomain redirect as provided by WordPress.com, and everything works exactly as it should for the blog.williamalba.com redirect. (Yay!)
You then advise me:
Once you do that, you should use your registrar functionality to redirect williamalba.com (and http://www.williamalba.com) to blog.williamalba.com, so everything will work as you want it.
Here's where I had trouble, almost certainly because of my lack of knowledge. In paragraph 4 of my original post I describe what I attempted:
I also created CNAME aliases for <www.williamalba.com> and <williamalba.com> to point to: <blog.williamalba.com>. However, visitors to those to URLs don't arrive at <blog.williamalba.com>.
On the night of January 3, uncertain whether someone would respond, I attempted to fix this myself by canceling the subdomain redirect and then installing a domain redirect. As you noted, this means that the entire domain williamalba.com DNS points to WordPress.com. Then on the morning of January 4, kardotim seems to have done something (I don't know what) so that all domains and subdomains of williamalba.com point to wlva.wordpress.com most of the time. I'm happy with that, for now, except for the most part.
That said, I'd prefer your solution, if you could please describe how I should go about redirecting williamalba.com and http://www.williamalba.com to blog.williamalba.com. The CNAME solution didn't work for me previously, even after giving the information two days to propagate.