I am answering my own questions here.
It appears that the hosted UI only interacts with the Couchdb, hence only needs port 5984 accessible.
Since my home network doesn't have a static ip, the fix was fairly simple:
1) On my router, I set up a static IP (DHCP Reservations) for the OpenAg RaspberryPi so that it always has the same internal address.
2) On the router, I did a single port forwarding (5984 -> 5984) forwarding anything coming in on the router to 5984 to the internal IP (and port) of my RaspberryPi food computer.
3) I used no-ip.com to create a DDNS (free DNS service).
4) I just gave the hosted OpenAg UI my DDNS, and it connected me up (which it had not done before I got the port forwarding for 5984).
There is no need for complex multi port mapping. This works as long as you don't have multiple food computers all using 5984 for Couchdb.