How to have form not require billing address if zero fee in priceset is selected in Event?
Sounds like https://issues.civicrm.org/jira/browse/CRM-16929.
You might need to apply the 3 patch that were submitted to this issue -


https://github.com/civicrm/civicrm-core/pull/7191
https://github.com/civicrm/civicrm-core/pull/7286
https://github.com/civicrm/civicrm-core/pull/7289


The first two are pretty longer patch so not sure if they will be applied directly. Recommendation is to upgrade CiviCRM to the latest version.