Claire was the one who broke the circle of ash around Jacob’s cabin.
When MIB disguised himself as her dad and she followed him into the jungle, he could have convinced her to do what he was probably unable to do in order to get to Jacob. Claire was probably "claimed" by MIB. She then abandoned Aaron and followed her dad (MIB in Christian’s image). He could have then convinced her to do what he was probably unable to do in order to get to Jacob. i.e. break the circle of ash and get into the cabin.
MIB wanted to be there in order to tell John what he must do. This break in the ash was what allowed MIB (in Christian’s image) to then instruct John Locke to move the island [Christian claimed to "speak for Jacob" when Locke visited the cabin] thus resulting in the creation of the loophole.
Jacob appearing to Hurley could have been MIB (in Jacob’s image) who intended for Saeed to survive, probably so he could have an ally inside the temple and thereby get to the Other Others / temple Others.
The fountain, is in my opinion, a “healing” water. That was why the Japanese guy Dogen cut his hand before dipping it in the water. He wanted to check if the water still had its healing power. I assume Young Ben was also healed at this same fountain.
Because the water was no longer clear, due to Jacob’s death, Saeed did not heal immediately. Had Hurley informed the temple Others of Jacob’s death prior to them immersing Saeed in the water, I think that they probably would not have taken the risk of him becoming ‘dark’.
Here is some of what I have heard about the Djinn:
The Djinn are a creation just like angels and humans.
The Djinn are created from fire (smokey?)
They can appear in human form.
They can be incredibly strong / powerful.
When a person wishes to communicate with them he must draw a circle around himself and not step out of that circle. Remaining inside the circle is a form of protection so that the Djinn can’t get to him / her.
If a person is ‘possessed’ by a Djinn and a third person wishes to communicate with the Djinn, the Djinn speaks through the host, but in an unfamiliar, deep voice.
The Djinn and place where Djinn live are not visible to humans.
Wearing a talisman can be a form of protection for humans (ankh on a necklace?)
I would like to hear other fans thoughts on these observations.