Could the appearance of Moses and Elijah with Christ at the Transfiguration in Matthew 18:1-8 be used as evidence of reincarnation?
No. The appearance at the Transfiguration of Moses and Elijah (symbolizing the law and the prophets) was to announce the fulfillment of the Old Covenant through Christ’s death on the cross (Jn 1:17; CCC 554-555).
The Transfiguration could not be a case of reincarnation. Since Elijah never died (2 Kgs 2:11), he could not reincarnate. There are alsohints in the Church Fathers that Moses’ body was reunited with his soul in heaven after his death.
While God can allow saints and angels to take the form of human bodies, this doesn’t mean that Moses and Elijah actually appared here in their physical bodies. This episode tells us only that they were somehow manifested to Peter, James, and John. It does not explain the mode of manifestation, nor does it even hint that Moses and Elijah had been reincarnated.