What happens if, try as you might, you are convinced the church is wrong. In this case the apparitions that the RCC says are Mary. If you are convinced by Scripture that these apparitions are not Mary does your differing viewpoint affect your salvation?
I don't see how the scripture can say anything about marian apparitions (except, theoretically speaking, the one to St. James). However, if, hypothetically, I find a disagreement between parts of what the Church teaches, for example, a contradiction between the scripture and some other part of the General Revelation, then I will conclude that I do not understand the teaching in question. If however, I had stubbornly refused to acknowldege my limitation and insist on my own interpretation of the revelation, then I indeed would have committed a grave sin which absent absolution would lead to my eternal damnation.