Studies in First Corinthians

So I was reading ahead in 1 Corinthians, and I came across verse 9 of chapter 2, where Paul quotes Isaiah 64:4. Only he doesn't. He - well, if it was some modern preacher, I'd call it a misquote! And it's not something as simple as "a" versus "the" or anything; he actually changes the meaning of the verse, and uses the changed meaning as support for what he's saying. It's not a vital support, but if some pastor today did this in a sermon, he'd be dragged over the coals for it! That is, if his congregants were paying attention...

I'm still not sure whether the third phrase in chapter 1, verse 2, is intended as part of the "to" or it modifies "called to be holy." As I see it, it could be read either way. I think the latter makes more sense - the letter was written to the church in Corinth, whose members, with everyone else who calls Christ Lord, are called to be holy.

How one interprets this verse will affect the interpretation of the entire rest of the letter, so it seems pretty important to me, but I haven't been able to figure it out on my own. :( Any help?

And why is it okay for Paul to mess with quotes like this?!