The Legion,
Lets look at the attack at the dam
Caeser sends his troops up the Intake tunnels, sneaking pass the NCR defenses and fortifications. The NCR was expecting to repell a frontal assault and the Legion swarms up from underneath.
He mounts a simultaneous attacks at Forlorn Hope and Camp Gulf, cutting off NCR reinforcements from those bases. The fiends assault camp McCarren handing the troopers there. While the Omertas assault the strip. You can hear about these events either from the epilogues or during the assault by listening to the radio. He also blocked the road leading to the Dam further cutting off the NCR reinforcements.
Caeser had a well organized and well timed series of attacks to strike at the NCR up and down the Mojave. Many people choose to ignores all these facts about the battle and fasley claim it was just a Frontal Assault. When in fact it was anything but. The plan is already in place, Caeser doesn't need to surive his tumor for the dam to fall. Without the Courier to save him he could still survive. Vuplis could kidnap a skilled doctor, Mitchell, Julie Farkas, Dr. Usanagi. And some elite centurions could probable handle Vault 34 long enough to reach the autodoc, they'd take signficant loses but its doable.
You're taking this from the end slides.. The camp golf attacks would be 'after' their ending has them take the dam..
The fiends attack a barely populated mcarran, the troopers had already left..
The legions do indeed sneak into the dam, but they are still poorly armed. Doesn't really matter how well you are trained, bring a knife to a gunfight and you aren't going to get very far..
As far as the Legion kidnapping a doctor? I guess that is possible, would show the Legion that their god was 'weak' though, might not bode well for morale or Casars future..
As far as the Omerta's go, in order to get this far you HAVE to have killed house. Which means that the Omerta portion of the ending is kinda nullified, with house around that battle would have lasted all of 30 seconds..