While I understand what you were trying to do; keep in mind, General Oliver is probably arrogant. No one gets to general by being nice, they get there by stepping over people and getting results. Also, as you are just a common courier, he would not take you seriously...though he should.

Even if you could (and should have been able to) negotiate a treaty wtih the NCR; House tells you at one point, if there were no Legion, Vegas would be absorbed or attacked by the NCR. Every conversation I ever had with anyone with the NCR always talked about Vegas svcking money and resources out of the NCR. This would be intolerable for any government. Hey, they ( governments ) all want taxes, and or control. I was trying to walk a thin line like you, but in the end, decided a rocket up his rump might teach him a thing or two about manners.

The NCR's leadership has become corrupt and hates any possible challenge to them. Thus, you having the securitrons that answer to you makes you a challenge. Tandi's gotta be rolling over in her grave.

The Legion? Oh, yeah, Caesar really is gonna like the idea of one of his ruthless lackeys having a suped-up pre-war death machine army. He's gonna expect you to turn it on him first chance you get.

Thus the reasoning behind you getting that army making everyone mad at you.

Ok... so how would those choices be better than being destroyed by said Machine Army immediately. If anything an immediate alliance would make more sense since it allows them to eliminate an existing threat and focus on the player's army afterwords.

The NCR is the closest thing to a fully stabilized government the US seems to have. While I wouldnt turn over the robot army Id have absolutely no problems taking orders from the NCR in how to distribute the use of the robot army to help secure and maintain order in the mojave and also to pursue the Legion and eradicate them without loss of any life from the NCR

You go with Yes Man and you make sure you have 100 speech. 100 Speech convinces Lanius to leave because the NCR isn't strong because it is too big to support itself... and you convince the NCR that they should leave you be. Altogether satisfying path. It means they're still at eachother's throats and neither of them want to mess with New Vegas.

That aside the NCR questline was bugged for me. I got to the President quest in two lines and the darn ranger wouldn't give me the frigging quest update. The voice acting also changed when he responded to me. Even more irritating. So 've got a save for when its fixxed so I can see the NCR ending.