Man, that's annoying, you just killed 20 off the 21 raiders, and Marcus pulls out his minigun and cut's you group in half. The first time that happend to me I shot Marcus on the spot, and then reloaded.
There's a third way, you can talk to the drunk guy in the small building on the Westin map. He'll tell you he made a map on a painting, but you have to buy it from the doc in the NCR.
I think it's inevitable, I once finished the game before Frank payed them a visit, but then the deathclaw's aren't mentioned. And I continued to play after the end and they still got slaughtered.
If you have a decent Science skill you can start the bomb without the key. BTW there are many way's to end the game, starting the countdown on the bomb isn't the only way...