Как насчет этого (опираясь на ответ Гамбо):
RewriteEngine On
# Redirect direct subdirectory/ request to main domain
RewriteCond %{THE_REQUEST} ^(POST|GET)\s/subdirectory
RewriteRule ^subdirectory/?(.*)$ https://example.com/$1 [R=301,L]
# Redirect www/non-https to https://example.com
RewriteCond %{HTTPS} !=on [OR]
RewriteCond %{HTTP_HOST} !=example.com
RewriteRule ^.*$ https://example.com/$0 [R=301,L]
# Redirect (internally) to subdirectory/
RewriteCond %{REQUEST_URI} !^/subdirectory/
RewriteRule ^.*$ subdirectory/$0 [L]