I agree 100% that Christ assumed our human nature, to say otherwise is heresy.
Here is a quote that states the crucial point of the IC, in my view: source http://newadvent.org/cathen/07674d.htm
The state of original sanctity, innocence, and justice, as opposed to original sin, was conferred upon her, by which gift every stain and fault, all depraved emotions, passions, and debilities, essentially pertaining to original sin, were excluded. But she was not made exempt from the temporal penalties of Adam — from sorrow, bodily infirmities, and death.
So, it sounds to me like she was restored spiritually at the first moment of her existence to the state Adam and Eve were created in. Therefore it makes a massive difference whether that state amounts to having a different nature to us, or if it is the same nature but wounded.
If the two states are different enough that they are two different natures, say "unfallen human nature" and "fallen human nature", and if the IC is true, then everything collapses. For, if Mary was like Adam and Eve spiritually, then she had an unfallen human nature. Therefore, the Christ born of her would have also had an unfallen human nature, different from our fallen human nature. So, we would not be saved.
Now, you have been stating that Christ took on fallen human nature. But if Christ had a fallen human nature, it would appear that would include having all the same "depraved emotions, passions, and debilities" as any of us, as well as the physical consequences, and that would clearly be blasphemy and heresy to claim.
Therefore, I would say there is only one human nature, damaged at the Fall, restored to Mary by the Immaculate Conception, transmitted to Christ by the Incarnation so that He was "like us in all things but sin".
However, I also have to grant that if there was no damage to human nature at the Fall at all, then there is no need for an Immaculate Conception. But I think only Pelagians would say that.
Feel free to point out all the errors in the above statements