I've been working on resolving this situation the way I want, but nothing is working!
I did all the required tasks, but no matter what happens, Papa Kahn refuses to acknowledge an Alliance with the NCR and the option to talk to Cassandra Moore about the same Alliance is not presented.
Out of desperation, I tried ALL the alternate strategies in the hopes of achieving my goal.
But no matter what, I can't get the Alliance to work!
It seems my only choice at this point is to find the console commands to override the ingame quest tasks to "Force" the resolution I want.
So far though, I haven't been able to find out what they are.
And until this task is complete, I can't progress any further in the game!
And when I searched on the internet, many people have had this exact same problem, that they wanted to align the Great Kahns and the NCR, but could not.
Seriously, this game came out long ago, I would have thought this would be fixed by the time I got around to playing it!
But I suspect that they stopped working on the game and the bugs were never fixed.
Edit: Thank you Fallout Wiki Staff, I finally found the codes here to advance the quest to my satisfaction. At least, I hope it's working. The mission has a reference to reporting that the Great Kahns were wiped out, but my report to Cassie Moore stated that Regis is in charge (new leader) even though Papa Kahn is not dead and they're on our side.
But I'm past worrying about minor errors now, as long as I get the ending I want, I'm happy!