Re: Is this a viable configuration - ER Poe5

sure, it's perfectly fine.

There are a couple of ways to go about setting everything up. Here's how I'd do it.
NOTE - I am assuming you have a relatively recent controller version (4.x or 5.x)

EXISTING CONTROLLER

1. Create backup file, then shut down.

EDGEROUTER

1. set up subnet2 on eth1
   1. if ER-5 has 48V PoE, enable on eth1
2. Set up DHCP server for subnet2

UC-CK

1. Plug into ER eth1
2. After booting, find what IP address it grabbed, and navigate there in a browser (https://ip.of.uc.ck)
3. You'll see two options (Launch Unifi Controller / Launch Device Management).  Use Device Management and upgrade the UC-CK to the latest version.  Also set a static IP if you want (or set the DHCP reservation on the ER)
4. After it reboots, run UniFi setup wizard, and load the *unf backup

BACK TO THE ER

1. If you choose to use DHCP reservation for the UC-CK, set that. otherwise, skip to step2
2. In the DHCP server for subnet1, add the IP address of the UC-CK in the "UniFi Controller" field

UAP

1. reset to factory defaults (either ssh in, or with a paperclip)
   1. if you go the paperclip route, BE GENTLE WITH THE BUTTON
2. UAP will reboot, pull an IP from the DHCP server, and learn of the UC-CK.
3. Since the UC-CK was set up with the backup from the old controller, it should relatively quickly re-adopt the AP, and reapply networks / overrides / etc. that you had previously set.