I think what you want for (a) can be found under Airport Utility > Manual Setup > Internet > set Connection Sharing to "Off (Bridge Mode)"

For (b) you can go to Manual Setup > AirPort > Wireless Tab > set Wireless mode to "Join a wireless network" and choose your network from the dropdown. Just note that when you do that, the Ethernet port completely shuts off, so if the settings don't take you'll have to do a factory reset to try again. If you want the Ethernet port to remain active (IE you want to make the AX into a wireless bridge along with USB & Music sharing), you can set up the AX to "Participate in a WDS network".

Just everyone please note the inefficiency of WDS!

The BEST way to extend a wireless network is to:
0) Space the wireless base stations so they cover the entire area
1) Setup a LAN with ethernet between all the wireless base stations (that means plug an ethernet cable into the airport's LAN port!)
2) Set the base stations with the same SSID / "Network Name" & Wireless Security settings
3) Put each base station on different channels (1, 6, or 11)
4) Have the base station with the WAN connection to your cable modem or DSL do "Share a Public IP" and the others do "Bridge Mode."

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