Each state has its own regulations and rules. Some of them will have a time limit, so that running a vigorous Democratic Primary Campaign to the end would preclude getting on the ballot in the general election.

I think that Senator Sanders will do one or the other, but not try both.

I assume he will run in the Democratic primaries but if he doesn't win that will be it. He has no interest in helping a Republican win.

But he will not make it.
The cirpirate top Dems leaders will give him hell.

Who in their right mind would even want to be a candidate for President of this ridiculous country? Expressing a desire to do so should automatically disqualify anyone who wants to take a stab at it. I'd like to think that Bernie Sanders is too principled to even seriously consider selling his soul to the highest bidder.