The Courier is the only person to actually go out and kill the fiend leaders. In every conclusion in which you don't do it, the fiends stage an attack on McCarran that either takes the camp, or the NCR takes massive casualties in the defense. Clearly those bounty contracts don't work out.
That's because the conclusions are based on the actions the
player took or did not take with respect to any given faction and/or quest. This discussion assumes the player was never involved in the first place, in which case I would say it's fair to assume that all the things on
both sides that are there for the player to do would have instead been covered by the respective parties in one form or another as there is no need to keep things in stasis for the player's benefit.
Case in point: when you get to the endgame and talk to either Caesar or Colonel Moore they both imply the assault on the dam is 'imminent', however you can blow them off and go about your business for as long as you please and the assault itself will wait patiently for your arrival. Were the player not a factor to begin with, the assault would have taken place
long before it ever does in a typical play-through as there would have been no need for the player to run around like a headless chicken getting whichever side he/she chose to support off its collective butt beforehand.
I have noticed that those supporting each side have decided that all objectives for their chosen side are automatically fulfilled while all those for the opposition are automatically unfulfilled, which is highly unrealistic. It's more likely that some of each group's objectives will be met and some will not, and depending on which ones those are one side will win due to the other having failed at a critical point.
Now, both Caesar and Oliver have tunnel vision where the Dam itself is concerned, however their lieutenants know that the real battle is taking place along I-15 and the banks of the Colorado River. Should the NCR manage to keep I-15 open their supply lines are assured and the Legion will be in an untenable position even if the dam falls, since the NCR will have taken out the Legion's west-bank outposts and be in a position for an amphibious assault on the Fort itself. If I-15 falls, even partially, to the Legion, however, the it's the NCR who has insurmountable supply problems and will have to withdraw from the dam or risk having their army destroyed.
The way I see things playing out is that the Legion's scare tactics initially work and keep sizable detachments of NCR troops deployed all over the place, however once the attack on the dam begins these get recalled and thrown into the fight. Whether or not this is enough to allow the NCR to hold the dam depends on what Lanius actually does without the player there to do it for him; if he is killed leading from the front (which is likely given his attitude towards combat and the presence of multiple veteran NCR snipers in the guard towers) the Legion troops will be demoralized and unable to capitalize on their initial advantage, while if he lives there is a good chance they will throw the NCR out of the dam and drive them back to at least Boulder City if not further. This does depend on when the recalled NCR troops arrive; if they get there soon enough they might even regain a foothold on the dam itself, while if they do not then there will be a brutal fight in and around the ruins of Boulder City. The recall of the NCR troops would force a recall of the Legion troops that are pinning them down, as doing otherwise would doom the assault on the dam and render these troops' presence in the rear areas pointless anyway.
For the attacks on I-15 and the Ranger outposts to actually affect the battle for the dam, however, they would have to take place ahead of that fight, which would telegraph the Legion's intentions and cause the NCR to shift troops to counter these assaults. That might well leave the dam under-defended, however at the same time the NCR has probably crushed the Fiends and thus freed up the heavy garrison at McCarran for use elsewhere, and those troops might be sent instead of ones at the dam. Timing would be crucial; if the NCR makes a major move on the Fiends (and without the game-shackles odds are good Hsu will do exactly that and the hell with orders) then the aforementioned assaults should be launched immediately if at all, if they do not then the Legion would be better served having their troops in the far South outposts come up the river and land near Boulder City in an attempt to cut off reinforcements and/or any retreat. Observers at Camps Echo and Golf will spot any such movement, though, which makes it quite a gamble given that deployment of a few sniper-types to the hills just East of BC would allow for easy picking-off of the Legionaries coming up the river in addition to any fire they take from Echo.
Short form: I think the Legion's initial assault on the dam would succeed to the point of driving the NCR troops back to the visitor's center and the access bridge, but at that point the NCR cannot be flanked and their sharpshooters will pick any attempted Legion advance apart unless the Legion is somehow able to land troops behind the NCR lines in the face of heavy opposition from McCarran, as well as Camp Golf and any other nearby detachments. This does assume that Caesar lives to see said assault, if he dies first then the NCR just might take advantage of the resultant demoralization and drive the Legion out of their forward base, if not the Fort itself.