I've just checked on A1 merc. I'm not sure if I'm right but I'll try to explain whole experiment.
First I've checked dmg of merc without weapon 75-80.
Later I gave her "lionheart" armor 20ED +15dex. Dmg 84-91. Difference 9-11 It is not 20% of 75-80 (should be 18,5-20) moreover that damage should increase because of 15 additional dex. Here I do not understand calculating. Maybe Blizzard use very small percents
Anyway later I use WW bow which has pure physical damage.
Damage was 172-306. With "Lionheart" 194-345. Difference 22-39. I think in this moment 15 dexterity affected higher dmg and difference is bigger. I'm not sure but after this experiment I think that damage from non weapon stuff is calculated according to base dmg.
For example your barb has base non weapon 100-200 dmg (just for easy calculation )
Fortitude armor add 300-600 dmg. If your barb with weapon has 1000-2000 dmg fortitude always add only 300-600.
I hope that my explanation was clear even if I did misstakes. If you have any item which has pure EDmg (for exaple forty or biggin's bonnet have no str/dex/ele dmg) try to check it again. I have no idea how EDmg from armor affect EDmg from skills. I can only predict that:
total dmg = (Weapon dmg) + (base dmg*EDmg from stuff) + (base dmg*EDmg from skills) + (base dmg*EDmg from auras).
Anyway highest dmg in game has melee sorc
I had 12-55K but I think maximum could be over 70 000. It was elemental damage ofc.