Trump sought and actively tried to subvert constitutional government and overturn the results of the presidential election. And what he could not do through the arcane rules and procedures of the Electoral College, he tried to do through the threat of brute force, carried out by an actual mob.
Case 4:
D - 9 votes, R - 8 votes, I - 2 votes
Democrat wins.
There are systems that solve the spoiler effect, such as:
In each system, the candidate preferred by the most people (usually a majority) wins.