The Formal Presuppositional Argument? Revisited

A while back I tried to come to grips with the presuppositional argument as formulated in a syllogism by one such presuppositionalist. There's been a bit of furor about it since then, seen here, here, and to a lesser extent here.

My treatment of the argument has been under criticism, and though I admitted in the original post that I may be incorrect, I still haven't seen a clear consensus on what the argument should be. Paul Manata suggested that the argument could be thought of as a biconditional statement, but then backed away from that suggestion. For the record, though, a biconditional version would look take this form:

1) Logic exists if and only if God exists.
2) Logic exists.
3) Therefore, God exists.

He then posted authoritatively that the argument was best expressed as a modal argument, of the form:

1) If Logic is possible, then God exists.
2) Logic is possible.
3) Therefore, God exists.

I'd agree that both of those are valid arguments. The biconditional form seems to me to be more expressive of the gist of the presuppositional approach, and it may in fact be what Dusman intended, but without specifying "if and only if," I can't know for sure. Incidentally, the biconditional approach is the easiest way out of the fallacious form that I found in what he actually wrote. So it seems strange to me that Paul would prefer the modal argument instead, which seems weaker.

If I may be so bold, I'd like to propose that both could be combined into one really solid argument.

1) If and only if God exists, then logic is possible.
2) Logic is possible.
3) Therefore, God exists.

Pretty bulletproof, right? I mean, this is an argument that I could see myself getting behind if I was a presuppositionalist Christian.

But, I'm not a presuppositionalist Christian. So I'm going to see if I can pop it anyway.

1) If and only if God exists, then logic is possible.
2) If logic is possible, then an entity cannot have two contradictory attributes (law of noncontradiction)
3) Logic is possible (necessary for argumentation)
4) God exists. (from 1 and 3)
5) If God exists, then he is infinitely just. (from Christian doctrine)
6) If God exists, then he is infinitely merciful. (from Christian doctrine)
7) God cannot be infinitely just and infinitely merciful (from 2)
8) God does not exist (from 5, 6, and 7)
9) Logic is not possible (from 1, 8)

Reductio ad absurdum. Let me know what you think- I really do think having a proper understanding of the presuppositionalist argument is important to understanding the Christian approach to rationality.