Is it possible to start the .DAT editor that comes with the DJI without connecting to the Mavic?

Is it a standalone exe? where can I find it?

If you're using the PC/Windows version of the DJI Assistant 2, version 1.0.9 on a 64-bit PC, then navigate to C:\Program Files (x86)\DJI Product\DJI Assistant 2\Assistant\. If you PC is 32-bit, then the directory will be C:\Program Files\DJI Product\DJI Assistant 2\Assistant\.
Find the program file called Viewer.exe and run it to see if that's the program you are looking for.....

P.S. You must have previously downloaded the .DAT files while the aircraft was connected and powered on. But the viewer can open them independently....

Good luck, and if you have a MAC, then you're lucky, but on your own ;-)