You are correct, q1. Sanders was a populist-socialist, so had a chance as a white male. Otherwise, the regional Establishment politicians - the Cuomos - of this party will face the fate of O’Malley - 2%. For the next generation of primary seasons, the only national Democratic nominees will be black, latino, moslem, female, or gay.
That is probably true for 2020.
But if there's 17 candidates, like there were for the Republicans this time, anything could happen.