"(1) Keeing the Law does not necessitate impurity
(2) Mary kept the Law.
Therefore
(3) Mary's keeping the Law does not necessite any impurity in her."
That's nice but it does not correspond with reality.
(1) The law states that a woman who births a male child is impure and must undergo the purification rites.
(2) Mary births a male child.
Therefore,
(3) Under the Law, Mary is impure and must undergo the purification rites.
That's the reality of the situation according to the scripture.
(2) Mary births a male child.
Therefore,
(3) Under the Law, Mary is impure and must undergo the purification rites.
The problem is in premise (1). You are taking it as a modal statement, "Any woman, without exception, who gives birth to a male child is unclean". But the Law does not say that or mean that. The Law is remedy for uncleanness, not a stipulation that uncleanness necessarily (such that even a divine miracle cannot prevent it) follows the birth of a male child.
-A8