Allow me to counter, and remember this is assuming that House is dead/doesn't do anything and that the courier doesn't intervene on either side
Spoiler
-McCarran is greatly under supplied and garrisoned, like most if not all NCR outposts. The Fiends outnumber the poor forces there and would do a HUGE amount of damage. If the Fiends' leaders are not killed by the Courier than they will either take the base or the NCR will lose a great number in its defense. This is supported by the NV conclusion if the leaders are not killed, in both Legion and NCR victory games.
-The Strip and outer vegas will also be lost. Bare in mind that the monorail will be destroyed which means that the city is now isolated from any hope of reinforcements. The Omertas will take control of the Strip from the other casinos and the tiny NCR force. The Kings take advantage of the situation and remove all NCR citizens and forces from Freeside. The Fiends control the West side completely.
-Now that McCarran is either taken or nearly destroyed, and Vegas is out of NCR hands, you have the Khans taking control of everything in the Northwest.
-Camp Golf is behind the frontlines of the fight and as a result te NCR used it as a dump for the worst troopers and raw recruits. This is told by many NPCs in Golf. If the dam should fall, Golf would be no match for a victorious Legion of elites, attempting to apply continued pressure after a successful battle. Camp Golf is seriously not a problem.
-The Legion takes the dam using greater numbers and tactics, while taking advantage of Oliver's incompetence. We can assume also that the NCR troopers have decreased morale (more so than normal, which is saying something) due to the assassination of the President. Without support the Legion takes the dam without a doubt.
-Forlorn Hope has no hope from Nelson's Legionaires and support from the victorious Legion coming from the Dam
-The BoS retakes Helios and parts of the highway during the chaos, no question
Mccaran- Wait wait wait, the NCR defenses make sure they have a chance (the giantass wall). Besides that you act as if the Fiends are some well organized fighting force. Hell no they're tripped out crack heads. The Power Armored NCR troops would have a field day with them, and I honestly don't think the NCR would lose. Even if they did lose the Fiends wouldn't hold it for long. The Westside Milita would have a major advantage and I doubt they'd sit on their ass and let the weaken Fiends retake over things. As far as the Omertas go, with the Gun Runners and ammo from House's robots the Omertas would still be weaken. And if the scenario is that House is alive then they get owned by the securtons. And as far as the Khans go, that's still not a battle breaker. Whooo they control an area the NCR had little influence in anyway.
Golf- [censored] troops are the overwhemling force, but don't forget who lives inside the Camp House, the NCR's number one snipers. Even IF Caser could get troops across Mead they'd still be weaken by wave after wave of human meatbag. By the time they got to first recon they're heads would be blown off. Even if they tried from the north the Ranger stations would snipe any offensive and reinforcements from Golf would be there soon. Also elite troops aren't unlimited he'd use those for Hoover more than anything.
Helios-They'd retake it after some casulities. They don't keep it in the long term though because of...
Forlon- The way Nelson is set up next to Forlon a massive Legion offensive is impossible. The few troops would be shot down by snipers. But even then, they can't reinforce because of Ranger stations along the river. I just don't see it happening
The Dam- True morale would be lower but when it comes to live or death men have a habit of fighting hard. It's not as if peaceful surrender is an option. Plus given that Kimball the warhawk came up with the Boudler Dam plan I doubt Oliver wouldn't use artilley to prevent more Legion troops coming over. Hell all of Casers elite taking on the Dam forces will probably win, but they'd be in no position to attack Forlon to prevent a counter attack. It would be a hard offensive taking back the Dam, since the legion are already weaken, and Oliver loves overwhemling force, the day would be won by the NCR in a day or two.