I'm just trying to get a handle on what you're saying. I think I disagree, but I wanted to make sure I understand you before I go off arguing against something that maybe isn't even what you're talking about.
I don't know Fruchtenbaum, but if he says that musterion has a different meaning for Paul that it does for Plato, then I disagree with him.
The Greeks engaged in mystery schools, and that didn't mean that they taught things that nobody ever knew before. That doesn't even make sense. It meant that the schools teachings were hidden (mysterious) to the people who weren't in the school. It's the opposite of open source.
Likewise, then, early Christians had teachings which they only taught to those who were already mature in Christ. Notice 1Corinthians 2:6-7. This is just one of many times in Paul's writings where he seems to stop himself and say, "hey, there's more that I want to share here, but I can't until you grow up!"
Mysteries aren't things nobody ever knew before, they're secrets a few people knew, and kept hidden, just as the often quoted Colossians 1:26 says.
I'll concede the following - 1 Cor. 2:7-8 is not asserting the Cross was hidden in the OT.
Paul is talking about the issue of the Body's "our glory" that "the Lord of glory" also made possible by the Cross.
It is basically Romans 8's "whom he called, them he also glorified" - which is also beyond the issue of Christ having died for our sins.