I think I have answered some of my questions.
1) Still wrapping my head around Kamailio <> Asterisk. I would appreciate any pointers what are the minimum settings needed to have Kamailio forward user registrations to Asterisk.
2) RTPProxy version 3+ supports SRTP and video. The FreeBSD is currently outdated but we are working on it.
3) I'm still not sure if I define a user's encryption=sdes on Asterisk if that will allow connection between RTPProxy and Asterisk to be unencrypted RTP?
4) I think I was thinking about this wrong, these sockets don't always exists and it doesn't really matter if they overlap. RTPProxy creates these listening sockets on the fly, so creating one for 0.0.0.0 should work to listen on WAN.