It’s a scenario that I really love to dive into and, unfortunately (based on what I’ve read), the answer to your question of who replaces Baldwin is a very boring one: Neville Chamberlain. There was conspiracy afoot within the Tories after the public ruptures of Rothermere and Beaverbrook, leading to Neville, his brother Austen, Samuel Hoare, and Chief Whip Eyres-Monsell all agreeing that Baldwin was inevitably going to go. The cabal solidified around Neville at the time and it was also understood that his leadership was most preferable to the press barons.
It would’ve been Neville. Now, what the younger Chamberlain brother does in ‘31 is up to more interpretation because there is some evidence that he thought about bringing Oswald Mosley into the Tory fold later that year (a move facilitated by party administrator and noted enemy of Churchill, George Joseph Ball). Not something I’ve seen done before and would be an interesting avenue down which to go.