Warning and Fatal error updating from 4.6 to 4.7 com_civicrm/admin/configure.php on line 115
It sounds like the civicrm.settings.php file may have been removed when the upgrade/downgrade was applied to your site? See Upgrading CiviCRM for Joomla in the section "Back up your CiviCRM database and settings file". Hopefully you have a copy of the file in your backup, and can restore it to the appropriate location.

CiviCRM recommends PHP 5.4 or greater, which might have caused the white screens when you tried 4.7. PHP 5.6 is the oldest supported PHP version at time of writing; as versions 5.3, 5.4 and 5.5 are no longer supported versions of PHP, I'd recommend updating your PHP if at all possible.