Papal infallibility means that only when the Pope speaks cathedra or dogmatically is he infallible.
Any other thing he says is subject to error.
Original sin essentially as I understand it is the belief that we are born in a state absent of God’s grace and this effect has brought death into our world.
This why we need to be baptized and born again through spirit.
I think the Orthodox get confused because they focus on the sexual act of Anne and Joachim and don’t understand how that act of reproduction can be more Immaculate than any other.
While the act was pure it’s not the act but what grace God gave through the act.
Some things to consider Anne and Joachim were barren it was through God’s blessings that they were able to conceive.
They were very pious and shunned by their community as they were seen as cursed for being barren.
They had sex out of pure love, openess to life and in obedience to God.
Where the Immaculate part comes into play is that God allowed the Blessed Mother to be concieved with out the stain of original sin.
Mary had no inclination to sin not even venially.
The Orthodox reject original sin thus they reject the Immaculate conception.
They too believe she never committed sin but reject she is concieved immaculately.