Certainly, Wolfe spent a lot of time considering the difficulty of discerning the truth, but I don't think that you can reduce that to "lies become truth". Even if you could though, how would that apply here, given that Weer doesn't believe the diary is genuine? Gold wrote a story, Lois bought it, and now the gold is real? Wolfe isn't that simple.
I think a better argument for your POV would be that Weer is lying about the diary being a forgery to cover up killing Lois etc. That would mean that most of Chapter Four is a complete fabrication, and maybe that's true, but I don't really believe it. I have spent some time thinking about why Weer is writing this book. The only answer that works for me consistently is that he is trying to find peace by reexamining his life. The things he avoids, the things he won't admit, are things he is trying to hide from himself. I don't see how he--or anyone--could commit outright murder and then try to pretend to themselves that they didn't do it. You might counter that Weer is simply trying to turn his lies into truths. But he's not at peace. Additionally, Weer himself provides the argument that coins are sometimes found in the river because of shipwrecks upstream. You make the argument that we KNOW the gold is real, but a few coins found here and there are no proof of the diary's narrative. Why do you think that "Chekov rules" would apply in this novel? Your argument about the tropiness of secret weapons seems cut from the same cloth. "Wolfe did it elsewhere so it must apply here" is just not convincing. Weer has just wrested Lois's gun from her, if he wanted to kill her wouldn't he just shoot her? Failing that, hitting her with the shovel he's already holding would make a lot more sense than fishing out his Boy Scout knife. The knife is associated, in one of his conversations with Lois, with Chesterton, who was known to carry a secret sword in a walking stick. I think the knife's symbolism is very different than what you suggest here, and speaks to a part of Weer's character that is genuinely good but which, given his circumstances, has never had a chance to come to fruition.