I started thinking about why the Others wanted Jack, Kate and Sawyer. If it's because they are seeing things and might be sick, or psychic, why isn't Sayid there? Didn't he see Walt? As far as people seeing things, Sayid did tell Michael in "The Other 48 Days" (I think) that he saw Walt, but like I really think that was more Walt than Sayid - Walt, because of the island, might be powerful enough that he can project himself to people that are not necessarily psychic.
My guess is that Jack, Kate and Sawyer/James have this power too but maybe not to the level Walt does. It seems the Others do think this might be genetic because they took Michael's blood right before they asked about Walt.
My other guess is that the "sickness", is related to psychic ability, or the island's power to enhance it, whether through its magnetic properties (which may or may not be gone now) or just its lack of any other types of interference or stimuli. This might explain why Rousseau's team went crazy (too many visions, projections, etc.).
Whether the "sickness" can be detected in the blood is something the Others may be trying to find out. Jack, Kate and Sawyer better get used to needles! I'll think of some Sawyer one-liners having to do with blood, needles, or medical procedures! Maybe he could call fake Henry Vlad or Bea could be Vampira - yeah, those are weak. (Actually, the hardest scene for me to watch was the now-famous blood drawing scene in "Three Minutes!" I had to turn away!)
OMG, I just thought of something else. What if the Others were planning to get Walt off the island as part of their experiment? If there is more than one possible cause of something, I think you would try to isolate the causes and try them one at a time.
Let's say the Others want to see if the ability to see visions, project yourself or other things, etc., is genetic or due to a "sickness" (blood draws), because of the magnetic properties of the island (the button) or Walt. This would explain why Jack, Kate and Sawyer are with the Others, why they let Walt go and why they eventually wanted the button to stop being pushed.
Fake Henry probably wanted the button pushing to go on a little longer once Walt left so they could see how much of an effect Walt really had. However, I think there are other parts of the island, or smaller nearby islands (maybe where the barracks are) that may have the same properties, or even more magnets or buttons to push. This might be why the Others weren't that concerned about the noise and bright light and still let Walt go. Even if they didn't know about the failsafe, they probably have a back-up plan. I could see fake Henry declaring, "Failing to plan is
planning to fail, Jack!"
And what if Inman wasn't telling the truth, or what if he was lied to? He sure seemed to have a "screw this" attitude and may not have cared what happened anyway. What if the "failsafe" doesn't turn off the magnet but just resets everything? The experiment could still go on, nothing would be destroyed, and my favorite guys would still be alive! (Note to Eko: whoever harms my beautiful bald Locke is in for a bruisin'. Yo.) And if the Others didn't know about it or just weren't sure about what it does, they have a back-up, so they didn't care.
Parenting the Lost Way
Oh, one more thing. I showed my boys (4 and 6) the first part of the finale with Jack, Sayid and Sawyer swimming out to the boat. It turns out my VCR went nuts and refused to record any more! They wanted to re-enact the scene in the bathtub. Johnny (6) wanted to be “one of the guys who stripped.” (Mommy seemed to like that part, I guess, although I was pulling for the trifecta, and when Jack did not go shirtless, I was disappointed.) He did not understand why they left their pants on because that would make it harder to swim. I was honest and told them I was disappointed too (because of the swimming thing, not because I wanted to see those guys mostly naked . . . well, ok I did, but I didn't share any of this with them!) but that they could not show naked people on network TV at 9:00 pm. Bobby wanted to be Charlie because he determined Desmond was crazy. I agreed and told him he’s not a baby anymore like Aaron and Charlie seems to be the next smallest guy since Walt was gone. But I told him that would not be a good idea. I didn’t explain. I told him he could watch Season 1 when he turned 14 when I will get him the 10th Anniversary Edition for Christmas.