If salvation in Christ is not lessened without the perpetual virginity of Mary why must one believe the Dogma or be lost?

I think you misunderstand the second answer: Even without the perpetual virginity of Mary, our salvation in Christ not lessened! It is referring to the hypothetical case that if it didn't happen at all, it wouldn't have affected the salvation. They are saying, maybe, it wasn't necessary for salvation (this must be a fringe opinion of a liberal catholic, you can ignore it anyway). This doesn't say, "without our belief in that, our salvation in Christ is not lessened.

The Marian dogmas are central to the Roman Catholic Church, it is uncompromisable. I don't think the best Catholic view would allow that it was even possible for Mary to have a marital relation, in other words, the second answer is not representative of the Catholic faith. You cannot have Christ and salvation without the Queen of heaven, Mother of God. Rejecting basic dogmas about her is equal to denying dogmas about Jesus.

Even the assumption of Mary, regarding her ascension to heaven is an uncompromisable belief. The doctrine of original sin was postulated to exempt Jesus, and make him separate from mankind. Similarly, it is necessary to exempt Mary from the same human nature, which could be tainted by her sexual relation if her immaculate virgin state is tainted. As, it can be assumed that original sin passes or is contracted by natural human sex and birth.

From this catholic page on the assumption:

Note: By promulgating the Bull Munificentissimus Deus, 1 November, 1950, Pope Pius XII declared infallibly that the Assumption of the Blessed Virgin Mary was a dogma of the Catholic Faith. Likewise, the Second Vatican Council taught in the Dogmatic Constitution Lumen Gentium that "the Immaculate Virgin, preserved free from all stain of original sin, was taken up body and soul into heavenly glory, when her earthly life was over, and exalted by the Lord as Queen over all things (n. 59)."

It can be postulated that rejecting the immaculate and separate nature of Mary, would also lead eventually rejecting the virgin birth of Jesus, in a slippery slope of logic. The assumptions of her nature are created only to justify the separated nature of Jesus.

To be able to address the stated question (and the implied question), we must first lay out some underlying premises.

First, according to Catholic doctrine any soul who dies with an unrepentant mortal sin upon their soul will be condemned to hell (hence will not be saved). This appears to be equivalent to what the OP means by "losing salvation"

Second, any mortal sin (a sin done with complete knowledge and consent and involving grave matter) is sufficient; the necessity of belief in Marian dogmas isn't special in this regard. It applies equally to murder, adultery, fornication, theft, blasphemy, drunkenness, or any other grave evil.

Third, rejecting belief in dogma either by refusing assent or actively believing something contrary is a species of heresy. As addressed in Why is Heresy a Mortal Sin, heresy is a grave sin because it is an offense against the virtue of faith which orients the intellect towards things beyond natural reason and heresy is a willful aversion from God because "by false knowledge of God, man does not approach Him, but is severed from Him" (cf. Summa Theologica II-II q. 10 a. 3 quoted on linked question). Furthermore, "no one draws near to God except by faith, because faith is a light of the intellect.... anyone drawing near by faith must believe the Lord" (cf Commentary on Hebrews C11 L2, quoted in linked question).

To sum, a willful rejection of revealed dogmatic truth (whether Marian or otherwise) would constitute mortal sin which if one does not repent of that sin before death, one would be condemned to hell.

In other words, any member of any Protestant denomination or Church whatsoever who has what the RCC considers to be valid baptism and marriage sacraments are still completely and hopelessly lost if they knowingly and deliberately reject say, the 'Perpetual Virginity of Mary' for example

Not necessarily or at least we need some further qualifications. The statements above hold clearly for Catholics insofar as they are members of the Church both sacramentally and canonically, that is, the recognize the authority of the Church. Protestants (more specifically anyone validly baptized yet not in formal communion) are members of the Church (broadly speaking) yet not canonically part of the "commonwealth" of the Church. The question then becomes to what extent someone who doesn't formally recognize the authority of the Catholic Church is bound to believe what the Church has declared necessary to be believed. This in turn depends on what their reasons are for not belonging to the Catholic Church. If they remain outside the Catholic Church due to no fault of their own, they wouldn't be held accountable for not accepting the authority of the Church for the necessity of those dogmas. However, if they remain outside the Catholic Church because they refuse to join despite some knowledge that it is the Church established by Christ or obstinately refuse to consider its claims, etc, then that refusal to accept the authority becomes voluntary in a sense.

but it seems a contradiction to declare that a person will be lost unless they believe a doctrine that has no salvific benefit

The assumption is that only things that have salvific benefits must be believed. What though is a salvific benefit or what does it mean that a doctrine has salvific benefit? This could be understood different ways potentially.

One way is narrower in the sense of doctrines which describe some aspect of how salvation occurred and thus perhaps they are necessary to believe because they are necessary for salvation to be possible. An example of a doctrine in this category might be the virgin birth or the death on the cross. The question would be why must I believe specific things about how God accomplished salvation -- is my knowledge of them required for him to save me? The problem with this is twofold: one, in that it seems to make belief of instrumental value (I only need to belief what accomplishes salvation for me -- what is in it for me?) and two, it orders all theological truth to Salvation. While Aquinas does teach that at least in some respects justification is a greater work than creation, he would point out that creation is first in the divine plan; that is God didn't create a world so that he had someone to save. God created a world, which needed to saved after creation.

The other way would be broader and as a result looser or rather it only analogously has a salvific benefit because it's not belief in the thing itself which saves but God that saves, but rejection of what God has revealed (as related above) would be sinful and cut a soul off from the life of grace and salvation. More precisely, it is not explicitly a Catholic dogma that belief is only necessary on things that have a salvific benefit per se -- it might be a view espoused by some Protestant denominations perhaps.

Perhaps for a Roman Catholic the perpetual virginity of Mary adds a depth of understanding to the entire plan of salvation, the nature of God, etc., and that is fine

In the specific case of the Marian dogmas (Perpetual Virginity among the rest), the dogmas were formulated to safeguard Christological dogmas. That is, Marian doctrine tends to develop to highlight and safeguard doctrines around Christ and his salvific work. The perpetual virginity for example was declared amidst the early controversies on the Personhood of Christ -- i.e. in what way is Christ human or divine?