I've had a problem getting this new blog template to work the way I want, but I have largely solved it. Or, more accurately, I've hacked around it. I had been hoping for a more elegant solution, but in the end I simply went back to Blogger's conditional template tags for my individual blog post pages.
That actually imposes less of a load on my server, since Blogger generates those pages as static HTML instead of doing all the PHP rendering stuff on the fly, and I can easily make (and have now made) the sidebar different for the different views of my posts. On the other hand, the PHP is simpler to work with because I would only have to edit the include files, instead of the main template, when I want to make changes. I'm keeping the PHP sidebar and footer for the home page, monthly archives, and for other pages as I deploy the new design around this site.
As I said, it's a hack. Still, as a total non-programmer, I'm not too disheartened. Even Dave Winer, who's been blogging longer than pretty much anyone—and helped invent most of the stuff that makes blogs what they are—writes:
I think we must all go through this rite of passage, the docs for Apache are so cryptic and inadequate. The design of Apache itself is weak. But it is workable, you know that eventually you'll puzzle it out...
I puzzled out a solution. That's no surprise. After that, the server is "a patchy" piece of software to start with.