1. When the quantity is big enough then it > quality.
2. Just take the Mojave Outpost? And what? Hold it? Yeah, that worked at Helios One when NCR attacked, I'm sure it'll work there too. The Mojave Chapter are surrounded by armies, they want to survive, not just go out and fight til the last man standing, going out is suicide. Legion has shown, through their Centurions, that they can take out power armored soldiers and Caesar states they can take out a Chapter too, but due to it's defensive position Legion would lose a lot of men doing so. But out in the open? Yeah, I'm sure the MC would survive an army who defeated them once from the west and an army who's defeated people like them from the east. It ain't that easy for the MC, they can't just stroll on out of there and kick ass and chew bubblegum.
Oh and here is delicious official material on what would happen if they attack during the battle:
Spoiler "During the fight for Hoover Dam, the Brotherhood took HELIOS One, inflicting heavy damage on retreating NCR forces, but it was a pyrrhic victory. Once The Strip was secured, Caesar's forces overwhelmed and eventually routed the Brotherhood from HELIOS One and Hidden Valley"
Yup, they sure seem to win if they go out into the open.
3. IIRC, during the battle of Hoover Dam, NCR has to deal with Legion, Khans, Fiends, Omerta and I think Kings too. They are capable of handling them during the battle when Legion attacks, why wouldn't they be able to handle less than 100 BOS members when the MC is surrounded by them?
I disagree with your number 1, and it has happened in history, with not as vast as a technological difference the BoS has over others.
2. As I said, if I was BoS I would take both Helios and the outpost. Taking the outpost cuts off supply line. Helios is on the verge of going online with a weapon that can fry anyone outside. Used at the right time...ouch.
Taking the mojave outpost I would ambush the next supply convoy. After that I would imagine NCR knows BoS has it. NCR won't be able to counter attack, with the legion, fiends and all the problems going on. So, I would expect an attack from the west, and defend it as long as possible, but trap the heck out of it so if retreat the NCR gets a nasty surprise. However, I would hope it did not come to that, as I do all this to mainly reclaim Helios to bring it online and to force a truce with the NCR who would be in a tough spot.
That is what I would do, because if you just sit in a bunker, you are now surrounded with no exit.
Do nothing, and die,or take action and force the situation into at least a chance?
As far as the legion, I don't claim the writers haveexcellent strategy, from what I see in game it is the opposite.
Heck, If I wanted to be real aggressive I would take out the NCR at Primm then attack the outskirts of Vegas. IMO, the NCR is not prepared for an assault. It would also be a surprise attack.