I think you are getting confused by references to different firmwares.
PANS is very locked down and limited in the flexibility to adjust settings. This is because a DWM1001C with PANS has FCC modular approval, you can sell it without needing to do the full suite of intentional transmitter testing. Considering how much time and money these involve this is normally a huge plus.
However this means that there are a lot of features and capabilities that the part has that aren’t available when running PANS. In order to gain the full range if capabilities you need to use other firmware that will give you the ability to use any setting you want and any positioning system you want, SS-TWR, DS-TWR, TDoA etc…
The down side (beyond the additional firmware work) is that in order to then sell the resulting system you will need to do all of that intentional transmitter testing.
Basically if PANS can be made to work for what you want to do then use it.
For how exactly the register values impact the settings, such as how the power register works you need to look at the DW1000 user manual. The power register is an odd one, it had 4 different power settings, each of which is 8 bits. Within those 8 bits there are two different fields for two different gain blocks. One of the first things I did was write some functions that converted back and forth between gain in dB and register values.