"What is the means by which grace is forfeited?" We have only known one means by which grace is forfeited, and this is the will. Heresy is not in the intellect, which knows, but in the will, which moves the intellect to assent. It seems that those who are infatuated with invincible ignorance place salvation in the intellect, where it cannot be, rather than in the will, where it must be.
When Sean speaks about the "six-year-old schismatic in the state of grace," the question must asked, is he able to exercise his reason, and assent to, or reject a truth of faith? If he is able to reason, and rejects a truth, then he is a schismatic; if he is not able to reason, he cannot reject a truth nor affirm it, and therefore is fully Catholic, as fully Catholic as the boy brought up in a traditional chapel.
St. Therese the Little Flower tells us herself that she "denied Our Lord nothing from the age of three." If it is true that the Little Flower had the use of reason at this age, then she was capable of being a formal heretic at this age.