Here's how I can prove it wasn't. When John the Baptist came on the scene, the Bible says he came "in the spirit and power of Elijah." But when asked if he was Elijah, he said no. And God would not take any random person, such as Seth, and have them come back as someone else, because He does everything for a reason- and that would go against His reasoning and His Will. Why?...
Because for one, God says we are to live only once, then die and face judgement. So for any human to live more than once would be to have God go against His own words. For two, Melchizadek had no mother or father- that would include in a past life, and the Bible says he was born to Adam and Eve. For three, bringing a tenth as an offering to someone is a form of worship, and the Bible would not promote us worshipping Seth- a mere human. For four, God says that He shares His business with His friends, and He did not share with us that Melchizadek was Seth.
There are only three humans that came back for purposes that we can know and understand- Samuel, Moses, and Elijah, and they came back as forms of light, not as we are. In order for Seth to come back in such a way, we'd have to understand the reasoning, how it fits into God's business. And it doesn't even make sense.