Can a church-approved Marian apparition turn out to be demonic, and are approved apparitions infallible?

Private revelations are never deemed infallible by the Church; infallibility applies only to doctrine. Nor are private revelations required for your belief. The only things required are the Church's dogmas, Scripture, and dogmatized tradition. So no approved apparition is ever binding for belief. That said, if the Church approves one, it would behoove you to consider it deeply, as with Fatima. Unfortunately Fatima has been so mishandled that most people cannot make sense of it, because pope after pope gave it his own interpretation: Pius XI sat on the request to consecrate Russia; Pius XII consecrated the world instead in 1942, which by the accounts of Sister Lucia meant only half the promised blessing and three more years of war, until he finally consecrated Russia in 1952; and later popes each read it differently. My point is that even a true, approved apparition does nothing on its own; it only bears fruit when the Church actually obeys what it says. Approval does not make it infallible or demonic-proof; it makes it worthy of belief, not a new dogma.