How does the quality of the Biscuits vary / change with melting point or slip melting point of HVF or HVO?

My first remark is that WHO recommend not to use partially hardened oils and fats because of the high level of unhealthy trans fatty acids.
Secondly I would not use the melting point as criterium for quality of biscuit dough and hardness of the biscuit. See also my contribution to "biscuit people" and/or my website fatsforfoods.
It is better to use solids values (SFC or N-values) . The higher the solid value at the (ambient) temperature of use the harder the biscuit dough (if you use crystallised fats in dough preparation) and the harder the final biscuit.
Of course are these parameters also depending on the fatlevel and the level of other ingredients.

It is better to have melting point of HVO for biscuits and cookies not more than 39-40*C as it will affect the metabolism of human body.  In higher melting point fat It may create fat bloom on the biscuits after it is baked and packed.  It will leave some white spots on the biscuits where consumer mistake it as fungus infested.  So it is better to have low melting point for production also and dipping also where it is healthy to use and quality of the cookie/biscuit stays fresh until its shelf life.

Entirely agree with Mr Lansbergen's comments above and we would all be wise to follow his advice.

From a process and quality viewpoint I would recommend use of palm oil plasticised to 26 /27 C.   This will give optimum in terms dough release from Rotary Moulders, minimise 'oiling out' of the dough and provide perfectly acceptable eating quality in the finished product.