I'm not sure of the rat party rules - but in an emergency, can't dem bosses nominate and give the nomination to anyone they want at the convention; someone off the bench?
The fact dem electors didn't vote for the nominee in the primaries should be of small concern.
Not sure of the convention rules, but I think the delegates are bound to the candidates for at least the first roll call vote. If nobody gets a majority, then delegates can change their allegiance.