Some don't have a row 13, some do. Some a/c skip a letter across the row ie: AC DEF.

I've boarded from the left, right and rear.

I've never seen lettered rows and numbered seats. I've never seen high to low going front to back or the equivalent in seat-naming. But then, I've never flown Aer Elbonia so my experiences are far from comprehensive.

The consequences are frightening. Imagine snagging 2B and finding it in the back of the jet when you board!