Without the sentinel you will need either a VPN or the UGW (CUGW or PUGW) on 2 public IP addresses. One for the media gateway the other for the signalling. I have never found a way around this the way you are trying to do it. Also, if you DO put the UGW on the public side only ONE phone will work from another Public IP. If you try and use 2 phones on that remote public IP they will not work. If you have remote phones (etc) nothing beats the sentinel.
