Point 1 already answered by ZAM perfectly.
Point 2: It was not necessary for Satan to be present to whisper to Adam. He could use
the help of other creatures having access to them. There are narrations speaking of a
snake and the like, who acted in the middle. Whether these narrations are authentic or
not, the space for a third person in between cannot be taken away. Whatever, the case,
what matters really is that he made them to eat from the tree, and not by which means he
used. Also there is a misconception among people that it was a single or a few occasions
in total, that Satan used to make his plot successful. But, in fact, it was a long
gradual process in itself, as implied by the word 'Fadalla' in Quran-7:22.
Point 3: Why do you think that his progeny has no ability to accept Islam? Here you are
going against the basic principle that Iblis is among the Jinn and they have a free-will.
With the exception of the Iblis himself, who has gone astray for the eternal, other
Shayateen of lower grade should be having the free will to accept or reject Islam.Also
consider this hadith:
It was narrated that ‘Abd-Allah ibn Mas’ood said: The Messenger of Allah (peace and
blessings of Allah be upon him) said: “There is no one among you but a companion from
among the jinn has been assigned to him.” They said, “Even you, O Messenger of Allah?’ He
said, “Even me, but Allah helped me with him and he became Muslim (or: and I am safe from
him), so he only enjoins me to do that which is good.”
Regarding the above hadith, there is a difference of opinion on whether Messenger(pbuh)
converted him to Islam or He only disciplined him. Even if the jinn did not convert, he
was made to do something which is against his very nature, which gives a hint about his
changing nature. Hope that clarifies it.