@Brain21 

To partially answer my own question:

Yes, the XE75 acts like a layer 2 bridge between the WiFi and the Ethernet LAN.  All devices - Wifi and wired - are on the same subnet.

So I need to change the 3rd octet on all my wired devices from .16 to .68.

I verified that this works.  I can ping wired devices from my wireless device and vice-versa.  Infuse on my AppleTV on the wifi netwk can access my NAS on the wired network, connect to shares, etc.

I set up my NAS for a static IP, so I guess only time will tell if the XE75 tries to serve out that IP to another device

  @Brain21 Somewhat recently I learned that in router mode our BE95 has a subnet mask of 255.255.252.0 and a massive DHCP range spanning three third triplets.

So for example 192.168.68.1 to 192.168.71.254.

I could not get IP Passthrough to work from my AT&T BGW320-500 gateway so I eventually set it back to Access Point Mode.  I would assume all three third triplets could access each other....

  @Brain21 

There are two ways to avoid conflicts in IP addresses:

1. modifying the range of IP addresses assigned by the Deco, to exclude the IP addresses manually assigned to your wired devices (more / advanced / DHCP server)
2. reserving, on the Deco, IP addresses for those wired devices.  (more / advanced / address reservation) By doing that, you may as well, if you wish, change those devices to use DHCP, and obtain info about subnet mask, gateway, and DNS, via DHCP

  @jzchen 

Given the subnet mask, the four third triplet are, indeed, in the same subnet, so they can access each other.

This subnet mask specifies that only the first 22 bits of the local IP address are used to determine if two IP addresses are in the same local subnet, the last 10 bits are not taken into consideration.

If you want, to can change the subnet mask, as well as the DHCP range.