You seem to have this granitic idea of the legion as a cultural monolith.
It really isn't honestly.
While their culture doesn't forgive failure they are also not fans if losing a war and getting slaughtered.
A perfect example would be the contraposition between Italian generals Luigi Cadorna and Armando Diaz.
Cadorna being the pig headed generalissimo and Diaz a more Forward thinking strategist.
Of course Cadorna had his merits and his failures in World war 1 was also due to politics (His staff never made extensive plans to attack Austria but had entire folders on how to fuck the french) but that is no excuse for the eleven battles of the isonzo.
Cadorna focused too much on the Isonzo and did not accept failure as an answer and it ended with Venezia almost being taken by the austrians and the army routing.
Diaz came in, galvanized the population and reshaped the conscript force into something that could stop and then turn the austrians around (Thanks to the entente bleeding the germans dry of course but that is war).
Caesar is of course Cadorna and his idea while successfull as the start is going to lead to failure.
Who the Diaz in this situation could be I do not know, could be vulpes could be lanius it's an open question really.
But on thing Caesar was right the legion cannot sustain itself the way it was during the second battle of hoover dam.
Perhaps a failure there might lead to it fracturing or to its rebuilding into a more fierce army.
It kind of is.
The Legion's whole shtick is BEING a cultural monolith.
In short, Lanius retreating doesn't make sense, because we saw from what happened to Joshua Graham how the Legion doesn't even spare high-ranking commanders. I think only Caesar can get away with retreating from battle. Everyone else would get crucified or worse.
Especially since Lanius treats the Legion higher-ups like shit, so if Caesar dies, Lanius losing the big battle is the only excuse they'll ever need to move against the latter and kill him, and given that most of his troops died on the Hoover Dam, they won't have any problems killing Lanius. Either that, or if Caesar's alive, he'll just kill Lanius for failing.
Getting back to the mod........
Y'know, this mod could have worked better if they cut out the bullshit. The main story with the Crusaders vs. NCR frontier vs. Northern Legion works fine. They didn't need Mothership Zeta levels of adding space battles and Enclave remnants if they weren't going to let us play as the Enclave. Just have the main three-way battle, and place a mcguffin in the middle. Like say, if you can't play as the Enclave, have an Enclave base working on some important tech in the middle of Portland that all three factions would want, like some kind of superweapon or technology that would be practical in a post-nuclear wasteland. The Enclave guys get killed by Super Mutants led by a general who used to be a big hitter during the days of the Master back in Fallout 1, and you can then have the NCR renegades, the Crusaders, and the Northern Legion fighting against the mutants AND each other over the spoils.
We didn't need snakes that you have sex with.
We didn't need space battles or Archimedes II.
We also didn't need to rip off EVERY SINGLE FPS CLICHE known to man.
All we needed was a reason for these factions to be here, and have something for them to fight over.
And since we couldn't play as the Enclave, we didn't need them here, either. FO4 cut out the Enclave entirely, and it worked better than just letting them be in the game but not as a playable faction. Either make them a playable faction, or make them part of the scenery or backstory, or just cut them out entirely.