Does the "Iron Will" perk add will bonuses randomly?
From the wiki:
Iron Will - §200 Soldiers receive a larger Will bonus each time they are promoted. (Adds between 2-10 extra Will increases per promotion on top of the default +2)
Does this bonus of 2-10 occur randonly or is it based on something like performance during the mission(s)?
After some further testing, it appears that the amount of Will received as a bonus during promotions is random, within the range of 2-10. This means you will always get at least +4 Will from a promotion, with the possibility of anything upto +12.