Don't quote me on this, but I believe the mechanics for PS are different from PD. First off, melee weapons are only restricted by the amount of STR required to use them, which contributes to the weapon's attack power in of itself. As a result of having a non-skill-based requirement, PS is just a statistical increase of the damage weapons do. Something of note is that PS is especially effective when used with bladed weapons who otherwise struggle to not glance against armor stronger than light mail. There's a breaking point for PS as there is for PD, but I believe it depends more on the weight and damage type of the weapon rather than a variable related to it's STR requirement, altering the weapons effect much more (or less) compared to bows. For instance: increasing PS above 8 or so will not benefit smaller weapons (1h, small 2h/pole) but larger weapons will have more rewarding returns in comparison (large 2h/large poles).
As I stated above, I have no specific statistics to back this up, but generally speaking having PS at 7 or above is good for any melee class, with ranged classes affording to go to around 5 plus or minus two depending on their WPF, skill/attribute min/max and weapon of choice (1h mace vs. small 1h vs. large 1h).
EDIT: Regarding your 2nd question, I believe that PS does work regardless of WPF, but its effects are diminished due to WPF's damage and speed increasing properties (if the equation for calculating damage were to be [base weapon damage X STR variable X WPF variable X PS variable = actual weapon damage])