There is another angle to his prattling: depending on what and how much he says, he may make himself be not a good candidate for corroboration. If he's not, DoJ won't offer him a deal, and if he's not offered a deal, he won't have to ponder whether to take it. That's a rather risky (all eggs in one basket) and bizarre tactic, but if he's indeed committed to not "ratting" on Trump, and if he's very confident Trump'll pardon him, it may be the one he takes.
Were I Stone, knowing Trump as long/well as Stone does, I wouldn't take such a risk because over the years, I would have seen first hand how readily Trump will turn on folks who literally minutes before were his friends. When it comes down to Trump v. Trump's friends, Trump does only what Trump thinks is best for Trump. With an election coming up, I think Stone's a fool to expect a pardon anytime soon.