1) they don't believe in it. The Theotokas was conceived naturally by Sts. Johachim and Anna, albeit in typical "prophetic" fashion, they conceived Her very advanced in age. Their Icon shows them embracing with a bed behind them. The Orthodox tend to have a different view of see and marriage than the Catholics, this relates a bit to original sin.
As far as I understand the dogma of Immaculate Conception. It is not about if the Theotokos was born of a virgin or not. It is about the Virgin Mary born without the original sin. I don't think Catholic believe that St Anna was virgin when she gave birth to the Holy Mother of God.
2) We celebrate the Dormition of The Theotokas, She died just like Her Son and the rest of us.
What about her body being taken to Heavens?
3)It exists, It's different than the Catholics, or at least St. Augustine hopefully someone will get more into this. I think Pope's Benedict, Francis, and John Paul II really saw no conflicting view of Original Sin, and may be stressing it like we do now. Either way, this is a long answer. I'll answer in a day or two, and get more into the other two points later if no one else does.
Looking forward for your answer. Thank you
Let me start off by saying that no matter what answer I give you, it's going to be superficial. I'm just trying to give you the general "flavor" of the way this looks at an Orthodox 101 level (and honestly, I don't think most of us ought be too overly focused on talking about the Immaculate Conception, original sin and Christ's victory on the cross should be our primary thing to talk about if we feel the need to talk). Unfortunately, I'm going to be pitting this off a (perhaps vulgar) Catholic view because based off the way you formulated the questions it's the easiest way for me to convey information. I'm not trying to be polemic. Ultimately you can't argue, think, or read your way to Faith so these things should be secondary anyway and merely based off all our experience in The Church (Orthodox or Catholic).
I think with doctrines like the three you brought up and the Filioque, some sympathy can be afforded why they came up. The West was dealing with different issues in the East and had a different tool kit in some ways. And as I said, I don't think the last three Pope's see a conflict with standard Eastern formulations, it tends to be the East who has reservations. Still, The Catholic Church has really not been able to officially formulate any of these doctrines to the Orthodox likings.
What needs clarification this:
The Catholic Church would have to say the Theotokos' conception by her parents was pure and holy, without a need for God extraordinarily to apply “the merits of Christ” to Joachim and Anna’s sexual act of conceiving their daughter in order to free her from “the stain of original sin.” Clarification is needed that Mary really died and was not assumed bodily into heaven before vanquishing death through her own death, by faith in her Son, Jesus Christ. So, in Orthodoxy it's affirmed that the Theotokos was Immaculately conceived by her parents, but it required no exceptional act by God to relieve her of the stain of original sin.
Everyone bears the consequences for the sin of Adam and Eve, most notably we die. Only they are guilty of that sin, we do not bear their guilt. If my child were to inherent a genetic disease from me, it's not because she's guilty, it is just the nature of sin and death. She would bear the consequences, but not the guilt of her ancestors. So when we get to the Cross, it's not about our Lord satisfying God's honour, or some guilt debt - it's about Christ's Victory over death, the final enemy, by trampling down death by death for His Bride. It's our duty to figure out how to deal with what we've been dealt.
Once again, perhaps the Catholics believe this too, I'm just giving you the basic "terms" of disagreement in the way I see it.