6. Your problem: cannot set up 'Next route on failure' properly. Besides destination proxy returns SIP error code 603 (Declined), PBX keep sending invites for aprox. 10 secs. 'till pass to the following route. Error codes are set like '400-606'. Any help deeply appreciated.
what is your ARS rules for failover like?
are both pattern-out for the call (primary and failover) in the same ARS rule?
maybe you need to update pbx to latest version.