The DNGC 8.4 should work for you, because, as you say the P7800 was supported starting with Camera Raw 8.3.
The issue you’re probably having is that when you select the photos to convert with the DNGC 8.4, you need to single-click/select the folder name, but don’t double-click/open the folder. If you just single-click on the folder name that contains your NRWs and then click Convert, the DNGC would convert all of them to DNGs.
Always use the latest version of the DNG converter that will run in your OS.
As SS says, the DNG works on folders only, not on individuals files. THe files are not supposed to "load". They will remain untouched and the DNG Converter will generate raw DNG files (copies) from each and every raw file.
You can then open the DNG files with any relatively recent version of ARC.