From my understanding, the Democratic National Convention was fixed more towards Hillary's favor, so it should have been Bernie against Trump. Sadly, I don't think that Bernie would have won, but he would at least have run against Trump. I think that the Democrats and Republicans both chose the worst possible candidates to face off against one another, two of which are perfect displays of what are exactly wrong with both parties. I think that if the Democrats went with someone else, then Trump wouldn't be in office. I remember hearing discussion about Biden running. Although I don't really care for liberal candidates, I think that he would have beaten Trump, and he obviously would be doing a much better job. I really don't see why they chose Hillary Clinton. As it is now, Trump has pretty much tainted the presidency for seemingly ALL of the future presidents (And he clearly doesn't care.). It's really going to be hard to come back from this one.
It is possible. A different person would have a different result, so it is hard to forsee everything. He would lack Clinton's baggage and issues. The most important being the email one and right or wrong rumors about charities.
Biden would have brought his own stuff to the table, but would have been a much stronger candidate in many respects. Biden is a likable guy.