Well, they were personal notebooks- so who knows how speculative he was being. The key thing is, this wasn't what he was working on when he died. Published works intervened between TWTP and his death. That combined with the sheer implausibility of the metaphysics you've described might suggest he wasn't that committed to the whole thing ;-). It sounds fascinating though.

He reasons: the universe is finite, it has a finite number of possible states, if any state was an end state it would already have ended, therefore it recurs eternally.

Are there any arguments for these claims? I'm fascinated by the (often very compelling!) arguments past generations had for how the physical world had to be. Aristotle is the best at this.