I un-merged the cell, set all the cells to unlocked, and then re-merged them. When I the modify the cell the procedure triggers and re-formats the merged cell to multi-line.

If I then view the properties of the cell on the protection tab the "locked" box has a square in it (not cleared, not checked).

When the sheet protection is put back on I can not select that cell any more (I'm assuming because of the square status in the Locked checkbox.