the truth is that clinton & trump are closer together than clinton & sanders are. sanders lost a three-way race early in his career, and never seems to have gotten over the guilt of being responsible for electing a republican. i don't know what the precise consequences were. but, he's still holding on to this. so, the circumstances are going to have to be extreme for him to avoid endorsing clinton.
but, his supporters are a different story. this is a different scenario than we've seen in a long time. past metrics are not going to be applicable.
what stein is going to need is a tipping point. there was a lot of mathematical research that came out about this around the arab spring. what was learned was that you only need to sway a small number of people to spur a movement. in fact, that appears to be what happened with sanders, too. he went from 10% to 45% almost overnight. it wasn't gradual. there's a lot of literature out there about this. stein's team should be reading up on it, because what it suggests is that she doesn't need to go after all of sanders' supporters, but just enough of them to get the tipping action.
http://www.theguardian.com/commentisfree/2016/jun/09/bernie-sanders-thorn-in-hillary-clintons-side#comment-76032206
http://uncyclopedia.wikia.com/wiki/Sociology