And from everything I've been told, the infant baptism is to ensure that a child is done, clean, slate wiped, incase they die in those grey years between being a baby, and being old enough to consent to baptism. It's not about the state they are in at birth. One view.
What does an infant need to be cleaned of if not original sin? You yourself say "slate wiped", "cleaned". That implies they are born with something wrong or at the very least they could sin enough to lose the ability to enter heaven before they were old enough to consent to baptism. Otherwise, why would we be spiritually "cleaning" an infant? If the church set an age at which children were capable of sinning and baptised at that age, then I could see them arguing they don't think babies are born into sin. But, infant baptism is clearly in place to cleanse them of an impure state. It doesn't say babies commit sin or are personally guilty of sinning. It does say they are born in a state of sin, a separation from God, inherrited from Adam that is washed away through baptism.
One of my favorite people on our unit is Sister Mary. She's an older Irish nun, and an absolutely AMAZING human being. Even she will admit that a lot of the things Catholics believe, do and teach are not consistent with church doctrine or the bible. They need to comfort their flock after all and mercy and compassion are just as important to some of them as following church doctrine to the letter.