To honestly answer the OP,
1: It doesn't matter. It wasn't Caesar crossing the Rubicon that made the fall of the Republic inevitable, it was Sulla's Proscriptions if not the class conflict fomented by the Gracchi. We've been living on borrowed time since the Great Society and our Republic has been doomed since the invention of the Twitter Mob. Maybe since the Bork hearings. The decline has taken time, but the damage has at best been halted by the victories of strong conservatives, never rolled back.
2: It would take a miracle.
3: Someone is going to be dictator. Dictators from the right with the very arguable exception of Hitler (The S in NDSP stands for Socialism) are generally not disastrous for their people in the first generation unless they get into an unwinnable war (which admittedly tends to happen). Dictators from the left with no exception I can think of bring ruin to their people unless their lives have already been thoroughly ruined by previous dictators from the left.
If anyone left leaning wants to push us back from the brink vote "not Democrat" all the way up and down the ticket. That's the highest polling non-Democrat in closely contested races and third party in effectively uncontested races. The only way out short of blatant divine intervention is if the Democrat Party polls third or lower in most races and enough rank and file Democrats coming to the same conclusion that the Democrat Party must be destroyed to do so nonviolently would still be miraculous. The Republicans would then need to follow through and impeach leftist judges and repeal civil service protections so that the establishment can be purged from the bureaucracy, which would take another miracle.
Things have gotten bad enough that there can be no useful discussion of issues until the Democrat Party has been destroyed. When a Green (or some other currently minor party of the left) can get into a presidential debate and a Democrat cannot only then can we begin to consider that the issues might possibly be about the issues not about excuses for the Democrat establishment to impose tyranny. Barring a miracle that will never happen. There will be no more presidential debates. Power will either fall to the chairman of the party central committee, who debates no one, or someone will be hailed Imperator to prevent that.
I think it is partly because most democratic systems are deliberately structured to be two-party affairs. Second is voters' tendency to vote "against" - basically, "I hate these guys and don't want them in power, so I will vote for these guys I hate slightly less just so the first guys don't get into power".
Few democratic systems are deliberately structured as two party affairs. Most use the parliamentary model, which encourages the formation of small king maker parties. Control of parliament is dictated by which of the big tent parties can buy the participation of enough minor parties into a governing majority, giving the minor parties disproportionate influence unless their demands are unthinkable to both of the large parties*. The American system was accidentally structured to discourage minor parties because the framers didn't think stable parties could organize across disparate states anyways. Only nations copying the American system deliberately chose two party rule.
Multiparty democracy is even worse than two party rule because without a majority party a coalition of parties representing less than a third of the MPs can hold control of the parliament hostage. Bidding for minor parties to join a coalition government takes priority over the platform of the would be ruling party. In the worst case there are two stable minority coalitions and one kingmaker party able to join either to create a majority. Preventing tyranny of the majority by enabling tyranny of the minority is not a good exchange.
There's
math, but it doesn't quite tell the whole story. Under normal circumstances, the two largest parties can not coalition with each other.
Penrose voting doesn't work for political parties because it can not handle two parties with the same platform. In the US that could take the form of every state having its own formally independent branch of each actual party
* the Bloc Quebecois for example wants the districts they control to not be represented in the Canadian parliament and if they get that they can no longer support the coalition that gives it to them so there's little or no incentive to court their vote.
