>Could a state democratically elect to secede from the union?

It was supposed to be part of the stipulations Texas had in the agreement to re-join the Union. At least that is part of Texas folklore. However, they were coming at it from a different direction in saying that the annexing was not done legally, so it should just naturally revert back to how it was before the illegal annexing.

We're actually seeing something similar but different currently in how part of Oregon wants to become part of Idaho.

>What if a National vote was held and a majority agreed to split up the USA?

It might not be a national vote, but a civil war ending in a stale mate would essentially do just that. Think Handmaid's Tale's Gilead.

> At what point could we democratically all agree we think the other side is crazy, and ask to peacefully take our ball and go home?

I gladly suggest donating Texas and Florida as the land of the new territory for the "crazy" for which you speak. Anyone not wanting to be part of there rules can move to the other states, and anyone wanting to be part of it can move in. Just let me move out of Texas before the mad rush