I am not entirely sure what you want to do.
I recreated it and got to the chevron shape:
But then you move the vertex downwards and end up with non-planar quadrilaterals:
I converted those faces into triangles, but is this what you want?
You really cannot expect to get the correct shape when faces become non-planar. That is, not all vertices of a certain face are in the same plane, some have been moved out-of-line. Converting to triangles ensure that faces don’t do this, as triangles are unable to have the same issue.
I don’t understand what you mean by valley shape, exactly. Can you provide an example of what you want?