Your transition 2 -> 3 is not justifiable, "doubt" is not the opposite of "think". A better parsing leaves "doubt" alone and would end with: "I doubt you are correct."
My dear, you just lost the "don't".
I'd be lying if I said I don't doubt you are not incorrect
I'd be lying if I said I don't doubt you are correct
I'd be lying if I said I think you are correct
Holy Shnikes, that's a tough one to parse! I wasn't sure what you were saying here, so I'm gonna break it down, working from the end of the sentence:
1. I'd be lying if I said I don't doubt you are not incorrect
2. I'd be lying if I said I don't doubt you are CORRECT
3. I'd be lying if I said I don't think you are INCORRECT
4. I'd be lying if I said I think you are CORRECT
5. I think you are INCORRECT
The idea is that each of the previous statements are saying basically the same thing; I'm just cancelling negatives each time. Anyway, am I correct to assume that you think that the GP is incorrect?