In order for me to fix this, I simply edited the templates so that the core content ended up inside the same set of HTML containers, using the same class name(s), as you'd have on the standard WordPress static "page" in Twentythirteen. If I remember correctly, that meant a couple minor changes, like updating the class name on a
div or two and adding an
<article> wrapper. The layout then fell into place naturally.
Isn't there a way you can leverage the default HTML structure of whatever theme and simply insert your content, as if it was just another posting, without having to worry about manually duplicating all those outer
div containers and their assigned classes?
As far as the reason why the content was so squished as shown in my OP: The normal padding that is added to the content area to accommodate the widget sidebar was applied twice; once to the content area as it should, and again to the inner content area, because the HTML was missing an
<article> container which would have blocked that CSS rule from also being applied to the inside content.