Most likely the standard, non dual-Avian version. The XBus version is ONLY for Spektrum RXs and uses the 4 pin Spektrum XBus connector. The dual Avian version is only for Spektrum radio and 2 separate Avian ESCs (not the Spektrum Dual 40A ESC - that needs the standard version)

The most likely reason is that it is not enabled. Reverse on the Avian ESC is a brake mode and you have to enable that brake mode using the Spektrum ESC Programmer (or you can use forward programming if you have access to a Spektrum TX and SMART Rx). Once enabled the default reverse channel is 7. The ETC can map any one of your transmitter channels into the channel 7 data sent to the Avian, so you can use any channel you want. You change this using the bootloader to manage the ETC configuration.

There are three possible reasons for this. The first reason is that the ETC is not correctly configured for your RX/ESC - are you seeing voltage or RPM ? if you are then the configuration is correct, otherwise check you have the right ESC selected (you might have to change the firmware for the ESC you need using the bootloader) and that you have the correct radio type and protocol selected. The second reason is that your ESC does not report current. Smaller or lower range ESCs often do not (for example the Castle Creations TALON 90 does not report current, neither does the HW 40A ESC). The third reason is that you have not reached the minimum value to start reporting. Most ESCs have a threshold that must be exceed before they will report any current draw. This varies from ~2A (Avian) to ~8A (Castle Creation). Until you go over that current nothing will be reported. If you are following good practise and testing with your props off then you might not be drawing enough current to trigger that minimum.

It might! Depending on the specific RX the Bind/Data/SRXL2 port is shared with one of the 4 pin satellite ports. If you use this for the serial expander it will prevent any satellite attached via the 4 pin socket from working. This is an RX limit on resources. If you use the SMART throttle port (channel 1) then there will be no loss of satellite functionality.