Purps, having been a catholic for about 25 years, brought up in confessional schools, etc, I seem to remember that the immaculate conception is referring to Mary herself, not to Jesus' birth, i.e. the Church teaches that only a perfect woman could have given birth to a god, so she is believed to have been born without the original sin, and in the same vein, it is taught that she never died, because death is corruption and she can't have been corrupted, that's why they celebrate Assumption of Mary on Aug 15th that is, her transfer to heaven before death, a bit like what the WTS teaches about Enoch.
This is not related to the very question you raise in your thread but, since I've no real valid opinion to express on the latter, this is my small take.