What is the ruling on one who prays Witr with the Imam, then stands up and adds one rak'ah so that his Witr is at the end of the night? And is his action considered departing from the Imam?

There is no harm if a Muslim prays Witr after the Imam has left, if he prayed Tarawih with him, so that he may make his Witr at the end of the night. This does not preclude him from being considered as having prayed with the Imam until he departed.