“...and was not blemished by the Fall, then she would have been otherwise fully fit for Heaven on her own, no?”
I think if the Latins are correct and there are such things as Original Sin and the Immaculate Conception, your argument holds together. If, however, there is no such thing as Original Sin, thus no need for the Immaculate Conception, as Orthodoxy teaches, then the Theotokos as a fully human person needed the destruction of of the power of death as much as any of us.
Orginal sin is passed down by the man, not the woman, who is only the carrier.
In Adam all die.
That is why no man could father Christ, since that would have meant Christ would have had a sin nature also.
The virgin birth was necessary so Christ could be the Second Adam and replace the first, so that in Christ, all are made alive (Rom.5)
Mary did not remain a virgin, but had a number of children after Christ (Mk.3)