So Jack asked Tom why they didn't just take Ben off the island to have the surgery done. His reply was something along the lines of, "Since the sky went purple..." which transmitted the idea that they can't get away from the island now for whatever reason.

So my question becomes, how does this impact Michael and Walt? They had very little time at sea prior to the sky turning purple.

If they could get off of the island before the sky went purple then why didnt they take Ben somewhere for help?
According to him he found out about the tumour before the plane crashed.
The sky didn't go purple to quite a while after.

I assume that Tom was talking about the electromagnetic pulse destroying a lot of the instruments on the island.

But the stuff on the Hydra island still works (~2 miles away?), so if Michael and Walt were that far away it might not have had any effect on them at all.

Perhaps the end of Tom sentence would have been "since the sky turned purple, we don't know if there is a world left out there"........did turning the key save the island and destroy the rest of humanity?

Michael and Walt are on island 3 waiting for their seaplane to land.

Third island, eh? :) Putting faith in the mittelosbioscience.com website? :)

If they could get off of the island before the sky went purple then why didnt they take Ben somewhere for help?
According to him he found out about the tumour before the plane crashed.
The sky didn't go purple to quite a while after.

Perhaps they were in the process of making plans to take him somewhere with more cutting edge facilities when the plane crashed, and those plans were cancelled in favor of having this world-renowned (?) surgeon do the job instead. If Ben has truly lived there all his life, he might be reluctant to leave the islands and would welcome a "miracle" solution fallng into his lap.
I know they said Jack wasn't on Jacob's list, but does that mean that he wasn't on Ethan's list. Maybe they wanted to get him all along, and when Fenry/Ben showed up, that was a desperate effort to "get" Jack because it was taking so long.