The power supply is compatible between the old Apex and the new Apex Plus. As long as it has 1.5 Amps at -48 V, you should be find, so our old desktop supply or any of the rackmount supplies will work fine.
What is critical is to not use the old PoE with the new Apex Plus. The same is true with the new PoE, it is not compatible with the old Apex.
With the new PoE, you can supply power to either side (management or data) and then run the appropriate cable from the PoE's management port to the OMU's management port and the same for data. Be sure not to mix these two cables as the two ports are different in the PoE. You should use STP for both cables.
You will still want to loop the cable that you aren't planning on using to power the Apex Plus through the PoE to get the protection offered in the PoE. Should something happen to the management power, the PoE will continue to power the Apex Plus through the data cable. For added protection, you can provide an additional power supply to the other power to prevent against a power supply failure.
For configuring on the bench, it is not required to use the PoE. You can directly power the Apex Plus via the connector next to the data port.
The table and equation to convert the voltage at the BNC to RSSI is provided on pg. 94 of the manual.