I completed Don't make a beggar of me first so now Melissa has disappeared and I can't complete Oh my papa. So what next - do I just slaughter the Khans or what?

You can convince Jack instead, as you only need to convince one of the two.

i just kill all of them. do it at night when they sleep. sneak, aim, split head and body.

Did you read the forum rules on not posting Spoilers in the General Section?

I suggest you do so please.

Moving this now.

I got stuck on this too. There is NO way to finish Oh papa. sorry. Even after starting For the Republic 2, I cannot get the dialog tree from regis to trigger the dialog with moore to allow the assasination of Papa, to then broker a deal. You pretty much have to slaughter the Khans. BIG bummer. Doing the beggar quest first screws all of this up. (horrible bug) Yes you can kill papa, but no dialog for a deal appears with regis, or moore.

You are thinking of Diane. (A little ditty about Jack and Diane...)

Melissa is the one near the Quarry Junction. If you do her quest first, then in Oh my Papa, Melissa disappears and you cannot complete the Papa quest.
What I did is wait until the NCR leader at the Dam gives you the Kahn quest. You have to convince Papa to quit the Legion. Since that is off the table, you have to either kill all the Kahns or
assassinate Papa Kahn without being seen and get Regis to assume command. He will fall into line without Melissa's input.

I thought I was stuck too. So I simply killed Papa. And talked to Regis. Went back to Moore and the Khans joined up.

I kill papa, no witnesses, and try to talk to regis, no dialog options to join NCR. Go back to moore, no dialog to even suggest it. Maybe because I did the whole karl journal thing before I ever got the NCR quest. Most factions I think it is okay to do the missions for first, and then do the NCR mission, but it seems like the khans are bugged.

How is it a spoiler to let people know in advance about the awful bugs in the game? Everyone should be told not to make the same mistake I've made.

Its a spoiler no matter what. There is no arguing that.

The same thing happened to me but somehow I got it to work though.