I have to defer to the product manual for this. As to what happens when the 620 goes down it appears the network goes down until the 620 comes back up.

If you want a mesh setup, time to change all this to mesh routers.

As to entering all the addresses, if this is how the 620 allow connections, then yes you do. Again, we hit the install and setup manuals (I didn't do this since that's your work) and follow the instructions.

Sometimes folk want the MAC address to be the same in a network. The answer is no. Why is in your second or third class on IP networking.