Airlink101 AP431W access point - only bridging first MAC seen

It's configured as an AP client.

Sounds like the AP client is working as expected, except that you have clouded the picture by introducing a switch and two other hosts. The "AP client" is supposed to support only one (wired) host. You cannot use a switch to expand the backend of the AP431W configured as an AP client. Perhaps a (wired) router instead of the switch would work, but that adds a level of network complexity that you way not want.

Sounds like you really should be using the AP431W in bridge mode, but then you will need another AP431W access point to build a proper wireless bridge.

Or you need to provide an access point client for each of your "devices". Or if any of these "devices" has a USB port that supports a USB-to-802.11 adapter, then that is another way to provide each "device" with wireless connectivity.

Edit:

Consider using a wireless router in wireless bridge mode. You may have to use alternate firmware like DD-WRT.
.