Currently, as of 3.0, it works that way, that as soon as you create a node, Full configuration poll is being done. If server is not able to ping AND connect to agent, the node will be marked at unreachable and the only thing that will remove unreachable flag and initiate communications is to manually run Full configuration poll again.
The thing is that after node creation, while user goes to properties of a zone to set that node as a proxy, the node gets unreachable. There is a way to create a node in a zone and set it as proxy for that zone in one step - in new node creation there's a checkbox "Create as zone proxy for selected zone". It's among other checkboxes and is easy to overlook. There are thoughts to improve user-friendliness of all this.
As for ZoneUIN parameter - as far as I know, it's related to SNMP Traps and Syslog monitoring; also it's used when tunneling agent connects to server so that server could auto-assign node to correct zone. Proxy should work without this parameter configured.