Thanks for helping me sort this! I did originally have a single URL in the config.php, but was having POST problems. Finding that hack seemed to solve things (excepting the new issues I began to encounter). Turns out that my problems resulted not from having multiple domains pointing to the forum, but from not rewriting http: to https:. I reset my config.php URL to the full https: address and added the following code to my .htaccess. Now I should be running properly.
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]