There is another way in through camp McCarran (NCR camp to the south of Vegas), you can take a subway from there so you won't have to go through Freeside. As for this key, I had it once too, not sure what is causing it. Could be something to do with not passing the credit check, or something akin to that, though I think after you passed the credit check once you should be fine later, even if you dropped below 2000 gold in pocket money.