You present a reasoned argument. But I need to clarify a couple of things. In your first lines
almost anything can be prevented ... long as you involve enough people
I want to point out that I only mean prevention by obtaining actionable information, not the idea that by keeping a guy off of the street we keep that particular guy from doing something awful again.
Also, under your numbered list of The only thing putting him in Gitmo does you left off one that I think is important. You are actually telling the world that we won't play fair with terrorists. (This is also, I think, the point that a lot of people are debating: that, in fact, we should play fair and take some sort of moral high ground.) We're telling the players that we can be quite nasty too.
And I completely agree with you that having Gitmo open "enables a lot of folks to ignore having this national discussion" and this could be exactly why we do it. Also, the idea that if we do get the terrorist threat to a manageable place we can close it down and "wash our hands" of it without having changed our stance in an official way.
But mostly I wanted to say that I pretty much agree with closing Gitmo. The idea authority without oversight is dangerous. But, in all honesty, if I knew it was working, I'm not sure I could be the guy to close it either. But that would be the only reason to consider keeping it open, and I have no information either way.