First, The Purge didn't kill *all* the on-island Dharmas. DL/CC acknowledged that the gassing that we saw didn't represent the entirety of the conflict between Dharma and Alpert's group. It may have been the death blow to Dharma, but it didn't extinguish them.

We know this because of what you said above. The Swan Station inhabitants were Dharma, not Ben recruits. Kelvin definitely survived the Purge, and may have even arrived after the gassing incident. The earliest Kelvin could have been recruited by Dharma was 1991. Probably the latest that the gassing incident could have been was around 1993.

The gassing looked as if it was limited to the Barracks, but we don't know that for sure. It was initiated at 4:00 PM, which means that they probably figured out when would be the best time for maximum casualties. The Swan Station, however, needed to be constantly manned. The button-pushers had their own bunks and their own food and such. Therefore, it's reasonable to assume that they were NOT part of the gassing, and that the Quarantine sign on the inside of the door was a result of that incident.

Kelvin told Desmond that Radzinsky killed himself. We saw Inman himself in moments of despair. It's likely that they knew that they were in the world's worst dead-end job. They were pushing a button that had to be pushed for an organization that had forsaken them. Radzinsky took the easy way out, and Kelvin must have felt like he'd won the lottery when Desmond washed up.

I don't know if the Others ignored the Swan because they knew that its inhabitants were too scared and too few to be a threat, or if there was some other reason they stayed away. They seem to have abandoned the Arrow, too. We know they used the Staff medical facilities and the Flame to communicate and the Hydra for "projects" and the Pearl to monitor things...and now we know that Ben had a use for the Looking Glass, as well.

But I'm with you on the food drops. That still confuses me. It's almost as if Ben kept up the ruse that Dharma was still functioning. Maybe he did it to keep away outside force?