This is the same thought I was thinking after
thinging for a while after posting.
I was testing my algorithms and wanted to "stay in the cell" in
order to try all the potential input conditions. I managed test
groups (hardware) for the last ten years or so. Unthingingly, I
continued to do so, then decided to move after an End-keypress. This
is normally not expected, so is clearly a "If it hurts when you do
that, then Don't do that" solution.

This should answer the rest of your questions.

The auto-filled text has only three possible values (Wide, Mid,
Narrow), however, depending on some conditions, only 1, 2, or 3 of
those are valid for a given situation (quite a complex algo). I sense
those conditions and limit the entry  to the appropriate ones -
displaying a MsgBox when user goes astray or suffers from fat-finger
syndrome. I also wanted to make things easy by auto-completing the
entries (fortunately the 1st characters allowed  this). No ammending
the auto-complete allowed.
I could have used Validation, and quite honestly, hadn't thought
of that until just now, but wrestling with modifying the Validation
list probably is no more complex than the array I search to find the
valid combinatins.
The End key  fires the Worksheet_Change Sub just fine, but Return
should normally be used exclusively. I also go to he trouble of
changing the "MoveDirectionAfterReturn" to "vbToRight" since data  is
normally entered in rows. And going back to vbDown upon Exiting Excel.

Hmmm... I'll have to understand this feature:
> If you want to edit the cell contents rather than move
> off the cell you have to press F2.

... But the end  key is a shorter reach on this laptop (:-)