Can you get the brotherhood to side with the NCR after you have already had the conversation with colonel Moore? She refused when I asked her if there was any diplomatic solution. I am nearly finished with Still In the Dark and do not want to re-load a save file and lose 3 quests I finished.

I havent done an NCR playthrough, but if memory serves, Colonel Moore, being a lover of war and killing as well as fighting the BoS back in California, doesn't want you to come to a truce. However, you CAN hammer out a deal, but ONLY if Elder Macnamara is still Elder, if you make Hardin Elder, he will obstensively refuse to side with the truce.

as long as you don′t overthrow McNamara, then yes. Moore won′t like it if you ally the BoS with the NCR (NCR infamy) but on the bright side: Nobody dies, and they are of great help in the Battle of Hoover Dam!