Slitherine

I know leaders seem to give an efficiency bonus and can give bonuses to attack defense. I'm trying to quantify it all better.

For example what are the mechanics exactly so I can compare a decision between buying 2 35 PP leaders vs 1 70 PP leader?

I guess while I'm at it I should ask if there is such thing as an overall efficiency formula, i.e. how do supply level, organization tech level, Unit quality, and leaders all affect troop efficiency in conjunction?

Also, once a unit gets -1 Quality because of MP issues how do you get it back to regular quality if your manpower later rises above 75%? I thought it used to be available as an upgrade but I'm at 77% in one game as UK and don't see the option of fix the quality on one of the GARs i repaired.