In case Brexit has already happened, can it be reversed after a short time?
Legally, the answer is no, I think.
But I can imagine a situation where, during the first few days after Brexit, all participants unanimously agree it should not have happened.
It is at least possible that the first days after Brexit create a crisis that makes all agree it went horribly wrong, and dangerously so.
From the EU side, let us assume it recognizes the crisis and is willing to cooperate in any way that its laws allow.
If all relevant politicians of all parties in the UK would unanimously agree that it lead to a catastrophic crisis, and should be reverted at any cost - is that possible in any controlled way?