Can't set 'Force Secure URLs'
You almost certainly want to upgrade your version of CiviCRM at least to 4.6, due to security concerns.

If you're already forcing https for all Civi pages via WP (or via apache/.htaccess, or any other mechanism), then there's no point in setting it in CiviCRM.

The base url setting is likely hard-coded into your civicrm.settings.php file, you'll want to change it there.

In general, having a consistent version of your domain is a good thing, for lots of reasons (SEO and caching are the two main ones).