To answer your questions:

1: I actually agree with you on this, and often suggest it as a solution when my partner and I talk about this. Although the twin that dies would come back as a heretic, (siphoner witch turned into a vampire resulting in a vampire that can use magic) which might potentially create a problem.

2: The coven lives on through Lizzie and Josie. That’s why the coven, in their final moments transferred Lizzie and Josie into Caroline. So the twins would survive and the coven would live on. So even though there is only two (or one after the merge) the coven still needs a leader. Perhaps there is some sort of side effect if the merge doesn’t happen, or maybe it will just happen because there is no “living” leader of the coven at the moment.

3: Kia isn’t alive, Kai is a heretic as he killed himself at Jo and Alaric’s wedding with Lilly’s vampire blood in his system.