Looking unto RCC's other 'sinless mediator and advocate' to advise the scriptural 'sinless mediator and advocate'.
Gal 1:9 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
It looks like you are saying that belief that Mary is a sinless mediator will send Catholics to Hell because Scripture teaches that there is but one mediator between God and man and that is Christ.
And--the truth is, I as a Catholic could agree with that----------------if----------------the idea that Mary is a sinless advocate were what Protestants thought it was. But Protestants do not understand what is being said and what is not being said when Catholics say that Mary is a sinless advocate--which is why they think the doctrine conflicts with Scripture.
I will try and flush out the doctrine in order to bring clarity.
Scripture teaches that there is but one sinless advocate between God the FATHER and man, and this is Christ. Note that Christ is a sinless advocate between God the Father and man---and that this is by NATURE. It is the NATURE of Christ as the only Son of the Father by which he intercedes on behalf of man to the Father. Christ saves us because Christ is sinless by nature, God by nature, but through the Incarnation, man by nature.
Mary is also a sinless advocate between man and God. However, there are some important differences between Mary and Christ and the nature of her mediation and advocacy.
1) Mary is not sinless by nature, she is sinless through Grace. Mary is sinless because she is redeemed, not because she is God, not because she is unredeemed, and therefore certainly not through her nature. Mary like all of us comes under condemnation. Mary like all of us could not be who she is unless she has been redeemed.
2) Mary does not intercede before the throne of the Father. Only Christ can do that. Mary intercedes before the throne of Christ.
3) Piggybacking on 2: the advocacy and intercession that Mary exercises before the throne of Christ is not something different then what ALL Christians do. All of us are able to intercede before the throne of Christ. That is what it means to be members of the Body of Christ. Note that the Church as a whole is sinless. Individual members of the Church will be sinless one day; the day they stand before God. There is no sin in heaven. What Mary has now is something all Christians will one day have. That is what redemption is: sinlessness.
4) Piggybacking on 3: The only difference between Mary and individual Christians is in the manner in which she was redeemed. Mary was redeemed more perfectly than any other Christian. This was because of the vocation God called her to. He equipped her with the necessary graces to fulfill that calling.
5) The means by which Mary intercedes and advocates is the same means by which all Christians advocate and intercede: God's Grace. Mary can intercede because the merits of redemption are within her. The intercession she exercises is that of Christ. Mary has no intercessory power apart from Christ. This is the same for all Christians.
6) The difference between Mary's intercession and that of individual Christians is only that her intercession is more perfect; but this is only because she is more perfectly redeemed. It is important to note however, that Mary does not do anything that individual Christians cannot do. Mary only does it more perfectly--and only because she is more perfectly redeemed.
When Scripture teaches that there is one mediator between God and man, Scripture has in mind Christ who is unredeemed, sinless by nature, God by nature, man through adoption, and intercession before God the Father and man. When Catholics teach that Mary is an advocate and intercessor, Catholics do not have in mind what the Scriptures have in mind with regard to Christ and his intercession. Catholics have in mind what Scripture teaches that all Christians do--except that Mary does it more perfectly.
Thus, Mary as sinless mediator does NOT conflict with what Scripture teaches about the intercession of Christ. If it did, then no Christian would be able to pray for and intercede.