You raise a good point. This idea has also been kicked around by Republicans AFAIK:
put up their own third-party candidate, attempt to block Hillary by keeping her from reaching 270, then electing their candidate through the House. This scenario, while awful in its result, would have the interesting implication of highlighting just how well and truly fucked our system is.
IF Sanders runs as an independent with Trump as the Republican candidate, that would be an incredibly short-sighted move that would pretty much doom the country to a unified Republican government. I would hope that he's smart enough to see that. HOWEVER, were Trump to run apart from the Republicans (or, as I noted above, were the Republicans to run their own "third-party" candidate), it'd be a much muddier situation. And I do agree with you that Sanders has a lot more independent appeal than Trump, but Trump also has sizable cross-partisan support. He doesn't just resonate with your stereotypical racist good ol' boy Southern white evangelical men, but rather
anyone with a strong proclivity toward authoritarianism.