A pinching of the lips will not produce a good valved bend. Valved bends can be done on the chromatic, as well as a valved diatonic. But, with a little practice valving your own harp will only take 5 or 10 minutes. For overblows, this means the blow reed is choked so as not to sound, and the draw reed is activated to produce the sound. Using overblows and overdraws it is possible to get full chromatic capability out of a diatonic harp, just as with valved bends.
Valves interfere with overbends.
For example, if a draw reed has been valved, an overblow is not possible in that chamber because the airflow cannot reach the draw reed during a blow. The bottom line is that you can't play valved bends and overbends in the same chamber.