This is just based on my feelings about Manchin – but I think it’s extremely unlikely that he runs for President 3rd party. Unlike Sinema, who I feel is a scumbag wannabe fame-seeker, I believe Manchin is a very shrewd politician. That doesn’t mean I think he’s a force for good – he’s largely been a force for corporate and (especially) fossil fuel interests, just not as far to the extreme as Republicans. But I do believe that he’s genuinely horrified by Trump and the modern Republican party.
As a very shrewd politician (likely with an ego to match), he loves attention. So he loves seeing all these people fawn over him and encourage him to run 3rd party. But he’s smart enough to know that he couldn’t win as a 3rd party candidate, and he has absolutely no interest in helping Trump regain power. So ultimately, he’ll announce he’s not running and endorse Biden, but probably at the last possible minute (meaning, likely, late next summer).
My feelings, anyway.