I'm lost when it comes to the food deliveries, but I think I have the Kelvin matter figured out.

I believe that Kelvin was one of the original Dharmas. I seem to remember him telling Desmond about "the hostiles". This at least indicates that, like the other Dharmas, he knew about the Others, but considered them to be dangerous. Sure the others could see into the Swan Station, but why not let a guy they were going to kill anyways continue doing the tedious work? Thanks to the end of Season 2, we know that pushing the button was an important task. As to the purge, I think Kelvin was probably unaware that anything had happened. This would explain why he continued to wait for his long overdue replacement in the hatch, and also why he tried to place the burden on Desmond.

There's only one thing I'm not sure about. Why did Kelvin say that it was dangerous to go outside? If I had to guess, I would say that this was just part of his plan to con Desmond into taking over his duties in the hatch. As long as Desmond believed that he couldn't go out, he'd be forced to stay down in the hatch pushing the button.