They have already raided as far west as Nipton. It's a stone's throw to the Mojave Outpost and the MSR. You cut the supply line and you can take the dam anytime you want. I wouldn't drive north from Nelson/Cottonwood Cove. I Drive west. You seperate the Mojave NCR from their reinforcements and you can defeat them in detail, one garrison at a time. All you gotta do is hold one pass. That's how I would plan the campaign. Drive west, take and hold the pass, wheel north to envelope Vegas them Isolate and attack the dam from both sides.
I see a couple problems in your plan, the Legion would have to capture the Mojave Outpost, they wouldn't get a large force there unseen, they got a small party to Nipton and razed it to the ground, and it wasn't unseen the NCR sends someone. (you) to find out what happened. Getting more then a small group there wouldn't go unnoticed. That narrow pass would be an easy place for the NCR to fortify themselves, while the legion gets intrenched fighting to take the pass the forces from Camp Gulf and McCarren would swoop in from behind and the legion would be the ones being flanked. Why is attacking a fortified river crossing a bad idea but attacking a narrow pass where the enemy can attack from California and the Mojave behind you a good one?.
Also Camp McCarren and Vegas can both be considered heavy fortifications, while your trying to capture them one at a time or together, and hold the pass.(assuming you captured it). The NCR sends the reinforcements south, then east before they head up the river and attack Cottonwood Cove. Or they blow up the fence at Searchlight Airport and move in through there.
Either way your entire plan to cut off the supply line went up in smoke.Lets remember that for a decade the NCR has held the Mojave without a second battle of the hooverdam, so getting troops around might not be as easy as you say. The dam is of vital strategic value. Other areas of the Mojave are strained because most everything is going to defend the dam. Take the pressure off the dam by sending a large force elsewhere and the NCR can reroute resources.
Here's another key factor the intake valve entrance might not last forever if the NCR discovers that weakness your secret tunnel into the enemy fortfication is useless. Via those tunnels the legion can
Blitzkrieg the dam via the intake tunnels take out general Oliver. Then their in possession of the fortress and the vast stock piles of ammuntion ment to hold a frontal assault from over the top. By this point you flush the intake tunnels and the NCR can either retreat or throw themselves at the fortification.
Its like having a secret tunnel into the castle's cellers. When you have that you don't [censored] foot around with a typical siege tactic, you use that advantage while you have it and take the enemy unaware from the inside out. Its not a fortress when you know the backway in.