>> this should be impossible according to Article 50

I thought it could be stopped at anytime with agreement of all member states. According to Lord Kerr (who devised Article 50):

"It is not irrevocable. You can change your mind while the process is going on.

During that period, if a country were to decide actually we don't want to leave after all, everybody would be very cross about it being a waste of time.

They might try to extract a political price but legally they couldn't insist that you leave."