The issue is direction. Themes call into Wordpress to get data. Mozilla plugins are called by Mozilla; if anything, Mozilla becomes GPL'd when it loads a GPL'd plugin. (Remember the whole "kernel taint" issue?)
Anyway, if you want me to dissect a Wordpress theme and show you why I think this, I will. But I think it's obvious if you look for yourself.
I don't think there is a case in which 'direction' figured in a copyright decision. The criteria are much more straightforward than that.
You never answered my question about what specific GPL language excludes the Linux kernel from the kinds of claims you are making about Wordpress. Or Mozilla (you claimed Mozilla loading a GPL'ed extension makes it GPL which is an interpretation that makes the GPL not 'viral' but an actual virus)
This isn't necessarily true - it's quite possible that a Wordpress index template is the first thing run. The idea that copyright law hinges on who called whom first remains preposterous, though.
Possible, but exceedingly rare. For a theme to provide a top-level script, it would be referenced with a URL like mysite.com/wp-content/themes/mytheme/scriptname.php, which would be ugly, brittle, and contrary to the clean-URL style that WordPress promotes. It is nearly universal that WordPress's code responds to a request first, and then dispatches to the appropriate files in a theme depending on the type of request.
Anyway, if you want me to dissect a Wordpress theme and show you why I think this, I will. But I think it's obvious if you look for yourself.