Essentially, this is it: Mary was one of a select few individuals who was given explicit promise by God that their seed was to be a blessing to the world. Obviously, as the virgin to give birth, she was the last such individual to be given that promise.
The first such individual was the Woman, soon to be called Eve, and then there were a few notable guys in between Eve and Mary.
The issue is this: with immaculate conception, Mary could have the same sort of relationship with her Son as Eve could, since there are unique biological legacies we get from our mothers that we do not get from our fathers. Thus the Seed of the woman in Genesis and Mary's seed need not be distinct.
But the presence of guys in the list really screws things up for immaculate conception. As the mother, Mary was these boy's only link to her Son and the only vehicle by which the promises made to them could be fulfilled.
You could not opine that raw humanity alone were sufficient (as could be argued in the case of Eve) since it is expressly stated that men beget children in their own likeness. If Mary is immaculately conceived then she was not born in the likeness of her own father and the only link the Son has to folks like David is imperiled.
Thus a protest against immaculate conception.
In solution, I've pondered if it is significant that, while we are told that men beget children in their own image as well as just as in Adam all have sin, we ARE NOT likewise told that women bear children in their own image in the same sense that they are passing on something like a sin nature.
What I've suggested based on trying to parse Genesis (which is an astoundingly astute text, so much so that it makes me wonder at folks who act like it's a mere story ... which implies that its specifics are thus of human invention rather than a record of divine origin) is that God MAY HAVE arranged things from the beginning to circumvent any issue and that women (or more properly the one who bears) may not pass on anything other than pure humanity to their children -- that the 'sins of the mothers' are not passed down as are the sins of the fathers (I would suppose as generic weakness or foibles concerning sin and sinning, so a father who trades in lust or lies may find that all of his kids struggle with similar weaknesses).
If what is true of Mary is simply true of all daughters of Eve, and Eve herself, then there is no need for immaculate conception (certainly not medically, nor legally and maybe not even spiritually — I smacked my head against my own limits when I tried to take it that far, essentially repeating the legal argument ... one reason I can assert that the true Person behind Genesis is a LOT smarter than me and I'm pretty smart as humans go). Likewise, Mary could be truly and fully her father's daughter and yet her Son would inherit all His spiritual nature from His Father, undiluted from what He had always had as the Son ... just now with full humanity too.
I should point out how this idea also negates any perceived need for all of the genuine heresies (like those listed, those against His incarnation) against which immaculate conception could be viewed as a DEFENSE (even though it puts the problem off one generation, from Mary to her father).
That doesn't mean it's true (how could I assert that when I couldn't complete a passably distinct argument for the spiritual level?), but it does mean it is at least a valid hypothesis so far as that goes.
As for your question, considering the attitude towards people teaching a 'different' gospel than that which was delivered once for all ... I'd say them folks are out of grace and in need of repentance big time.
That actually wasn't the question. The question was asking folks how they would resolve the apparent change in attitude from extra ecclesium nulla salus to the attitude currently expressed in the Catechism, as stated by the Msgr in the video.
As to your response, though, I agree that if somebody receives another gospel than that which was delivered to the apostles, they need to consider their actions, for even Satan disguises himself as an angel of light. That would apply whether the "apostle" is Marcion, Arius, Nestorius, or somebody more modern.
I would suggest that there are issues that have rendered the question as one relating to a moving target: the strong traditionalism of the Roman church being an issue on several different levels.
Actually, I see the Magesterium (the teaching authority of the Church) as being an anchor. A set of lines that keeps us within orthodoxy. Lose the Magesterium and you end up with a whole host of diverse and contradictory ideas: all of which claim to be Inspired Truth.
I mentioned Marcion earlier. He had some interesting ideas about God. Thought that there were two separate Gods (a Creator God and a Redeemer God). To support his ideas, he emasculated the Scriptures: he rejected all old Testament ideas, all of the so-called "Catholic" epistles (from John, Jude, Peter), and all of the Gospels, other than the Gospel of Luke. Of course, there was no set Canon of Scripture yet, so it was considerably easier for him to do so. Was he wrong? There were thousands in his day (and for a few hundred years thereafter) who did NOT think so. There are a few, to this day, who think he was right. But orthodox Christianity, as determined by the Church Fathers, based upon the teachings of the apostles (remember, Marcion lived in the early 2nd Century...the Canon of Scripture was not formally set until the 4th Century). In other words, his heresy was decided based upon Tradition (big "T" Tradition).
Another good, historical example, I mentioned earlier: Arius. Arius believed that Jesus was God's highest creation, but that he was a creation -- not God. Arius got a high percentage of Christianity to follow his teachings. In order to determine whether he was right or not, Constantine had to work with +Sylvester to convene an ecumenical council (the first since the apostles convened, as documented in the Acts). Again, the council made its decision based upon Tradition (as both sides of the argument claimed Scripture could back up their arguments). But if you look, today, the Jehovah's Witnesses would pretty closely line up to the Arian view of the Godhead. On the other hand, there is the view of the Oneness movement (as in Oneness Pentacostals).