[ealgeone]

Still, how do we know that Mary is an exception to the norm of “all have sinned”? And more specifically, is there biblical support for this claim? Yes, there is much biblical support.

Not per the Catholic Encyclopedia...."No direct or categorical and stringent proof of the dogma can be brought forward from Scripture."

https://www.newadvent.org/cathen/07674d.htm

For the dogma to be true it would contradict the numerous places in Scripture that do note all have sinned.

There are no carve outs for Mary in this regard. Not. One.

[Ultra Sonic 007]

From the same article you reference: "[Mary] in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin."

It is because of Jesus Christ that Mary was preserved (all for the sake of His glory), and not by any merit of her own. There is no contradiction.