Using sub domains

2 things

1. It did not land correctly. It still showed the domains landing page as stated instead of the sub domains landing page.

2. I have no idea what " lopping the rewrite rules under your "# SEO URL Settings" rubric" means.
Lopping generally means to cut or remove something..... should I remove something?

1/ Where "I did try this and the landing page worked, with the domain landing page showing" but "subsequent pages were 500," that seemed to mean that the landing satisfied you but subsequent moves gave 500. That led to editing (next).

2/ Yes, back up .htaccess, then edit that. In your .htaccess those redirects (listed above) are below the line "# SEO URL Settings" which accordngly is a commented silent "rubric" (sectional heading). With the particular rewrite rules lopped, dropped, cut, deleted, out, gone, THEN retry and see whether the 500s stop.