How do you think the game would have panned out if the Brotherhood held Helios One?

Brotherhood would have activated Archemdedes and held Mr. House and all of New Vegas hostage.

That is, if archemedes wasn't so nerfed.

And if we take the courier completly out of the mix, as he would have tipped the scales one way or another.

They would hold what is still widely regarded as a poor position for a military defense and would be a lot of men down too-boot; the NCR would come back.

The BoS didnt lose Helios because they didnt have Archimedes, they lost because of manpower, and if memory serves, they gun wasnt in their posession based on dialogue in the game. So it still would the same as giving your army 50 cases of missles and no launchers.

Can't see it happening.

I mean I can see them holding it, sure, but the BoS' problem is they're unwilling to compromise in their goal to posess any and all tech. They must know and/or they'd catch wind of the Securitrons on the Strip and wanna take action. That or they'd want Hoover Dam. Eventually they'd bite off more than they can chew and lose it all.

You don't need the range finder to activate the defense system which would have wiped out the NCR attacking the facility. And I would't discount the possibility of buliding a new one either. With the threat of Caeser looming the NCR couldn't afford the men and resources to take control of Helios One, it leave them vunerable. Its the same reason they didnt want to directly confront House and the Three Families.

The NCR would of made a Truce with the BoS. I could also imagine Frumentarii trying to infiltrate Helios many times over. Alot of Focus on the Dam would be redirected to Helios one. The NCR probably would make it a top priority to Destroy the BoS finally, and destroy Helios one with Explosives.

or they would capture it like they have now?

Honesty instead of being holed up in Hidden Valley they'd just be holed up at Helios 1. Really aside from a alliance with the NCR nothing really changes the fact that the Mojave BoS is never going to last.

I could only see it as being possible if Helios One was located on the top (or the side) of a mountain or large hill with the Brotherhood being surrounded by and NCR camp blocking all paths off of the mountain.

I think that would actually be cooler than the BoS in Hidden Valley and Helios One under NCR control.

Now here is the thing, if regular NCR troops can defeat the Brotherhood, I can imagine what a couple of Vet Rangers with AMR's and C-4 can do.

Tactics and superior firepower have nothing to do with it, the BoS had the skill and the firepower, what they didnt have was the numbers. In the words of Mr. House 'superior firepower and shiny power armor don't count for much when you're outnumbered 15:1'

Previous lore told us that small arms fire doesn't make a dent on Power Armor, and regular NCR troopers carry Service Rifles 98% of the time. So I guess the new lore tells us that Power Armor really isn't as invulnanerable as according to previous legend.

Nothing is bullet proof, merely bullet resistant, and I'm sure there's more to it than that, the point of the story behind Helios is the BoS were to arrogant in their toys, failing to realize having the upper hand technically cannot last long. It's not to different with a wasp versus an ant colony. The wasp is undeniably the stronger one, but the wasp, much like the BoS, is a living being, they can only stand for so long before they wear out by the ant colonies innumerable forces. The BoS likely lost strongly due to do fatigue and being overun. It's only a matter of time before the wasp is over run by hundreds of ants.

That's cause they nurfed it in 3 and nv

The best the BoS could have hoped for was to access and use ARCHIMEDES 1 to kill the NCR surrounding Helios 1. All the NCR would have had to do, though, is turn it into a classic siege and it would have just been a matter of time before the BoS ran out of supplies.

Sure, but bulletproof glass is also bullerproof. Doesn't mean it won't break if shot enough times. It's powered armor, I'm sure it's not pretty if it's powered system gets screwed up.
And doesn't mean it will stop EVERY bullet. .50 Cal for example is definitely getting through bulletproof glass, and I bet it would leave a mark on Power armor.

The BoS still need to eat and I don't think they'll be able to grow much food around Helios. The NCR could just surround Helios and starve the BoS out.

Plus Helios is solar powered so I guess the NCR could just wait until the weather gets bad.

The difference is that losing Helios ONE left them shellshocked. They were surprised by their loss and thus retreated and hid themselves away. If they won, they wouldn't be shellshocked, so while they may not expand immediately, I doubt they'd hide themselves either. With Caesar, House and NCR all in the area and all wanting them dead, along with the Brotherhood making goals of taking the Dam and taking Vegas, they wouldn't last much longer.