Forcing prisoners to run towards enemy fire sounds like something the Legion would do, not the NCR, and the Boomers do have farms and clean water, the NCR tried to cut off their water only for the Boomers to blow up NCR water pipes, which was very effective.
Also the NCR doesn't have all the time in the world to wait for the Boomers to run out of ammunition or other supplies to take them out, the Legion is still very close to attacking the Dam already in FNV.

that's a tricky question, well there's some factors to take into account Lanius has his camp extremely close to the Dam, i'd say that's a point for the legion, the legion agent in camp mccarren wouldn't be discovered without the couriers intervention, so the monorial would be destroyed, another point for the legion, if the courier died then benny would never try to escape meaning he'd never get caught be caesar, meaning they'd never have the chip, so no change there. so i'd go with a legion victory with heavy losses

i still think the legion there braking down the ncrs moral and just killing them one by one the legion grow by forcing every man in to the army and having women make more and more numbers. the legion grows the ncr shrinks so i just dont see the ncr wining but benny my take couriers place in some endings like wild card

Honestly, the Legion will end up in control of the dam regardless of whether they take it in the second assault or not. Simply put, when Kimball is assassinated, the NCR has no reason to continue pushing for control of the Mojave, Kimball is the one organizing the takeover and most other senators would rather leave the frontier alone.

With Kimball dead, eventually the NCR would have to draw back, leaving the dam to the Legion. That or they'd make themselves an easy target by reducing their numbers / strength there in accordance with bureaucratic ruling in Shady Sands.

One thing I loved about lonesome road is ulysses is very accurate on all the factions. The legion would inevitably take the dam regardless of the help of the courier, this is shown by NCR's lack of strength to even grip the most simple of their outposts. The legion have infiltrated the NCR deeply, this is shown at camp McCarren. The NCR would fall and be forced back west and the legion would pursue them. They have nothing to backup their money so that in itself is not viable nor creating stability among their citizens or society. Losing their president, their leadership will fall once the legion pursues them all the way back home. Upon the legion reaching the end of the NCR it would turn on itself because as war strong as the Legate Lanius is he's not capable of leading the Legion past a war and Ceaser is already going to die.