Is a corner counted as frontage in the above diagram? It looks like it could be 'seen' if there's no overhang from the other unit.
A corner is a corner, it is not a frontage, be it front, flank or rear. You cannot charge a corner.
I'm not going to add any diagrams, but will try to explain.
Say for example in the diagram that has been posted that the flanking unit consisted of only one stand, and that there was a third enemy unit that wished to charge and that it could only see the corner of the enemy unit between the two friendly units. In fact it cannot see said corner, as it in game terms occupies no space, has no facing and so on, is merely a reference point. So, said unit would not be able to charge.