I hope we all agree that a one party system is very bad. Every one-party nation has gone to ruin. In that regard, the invincibility the Democrats long for, as they have achieved in California and Chicago for instance, would be a disaster nationally; and irreversible, given the sleaze and corruption politicians are willing to employ to stay in power. For that reason alone, Open Borders and Non-Citizen Voting policies should be recognized as steps toward the catastrophe Democrats wish for.
As for your main theme of multiple parties, in 1992, we wound up with a president who won only 43% of the popular vote. If other parties are to have greater influence, the risk of a minority president will inevitably rise.
There is only one way (in my opinion) that additional parties can realize more than token votes: a runoff election of the top two popular vote getters, in the event no candidate gets a majority of popular votes in the main election. People could then freely vote their consciences without worrying that we might end up with a president who only got 1/4 of the popular votes.
Given the divisions in our society, such a runoff election would wind up being a permanent feature. The election that includes all parties would come to be considered a pre-election to sort out the candidates, with the main election on the first Tuesday of November. There is nothing in the Constitution that forbids this structure, as long as the electoral college is maintained, as it should be. [My reasoning for that can be found here https://medium.com/@rickfischer/part-of-the-problem-is-that-our-electoral-system-uses-districts-to-determine-the-presidency-and-6a1cd8aad67f#.ecxrbd2le]
As the Republican primary circus demonstrates, that would be a sound practice for the Parties as well.