1) There is a global setting called "support_update", if you set it to false the PBX will use INVITE (see http://wiki.pbxnsip.com/index.php/Global_Configuration_File on how to set it).
2) Yea, I agree that the audio with port 0 is really strange (well, that's how we get it!). Maybe Broadvox can try with the INVITE and if that does no work we can make a version that takes the audio part out in this case.