Question about element weakness

You are correct when it comes to mages. It does affect DoT too.

I remember it used to work differently for archers.
I'm sure it works differently for paladin charges.

Well, it "used" to work differently for elemental charges, but that's no longer the case. Elemental charges gain the full 150% weakness bonus and 50% resistance bonus now. It used to be some formula that made is so the charges didn't gain their full elemental benefits until they were maxed out, I can't remember what it was though.

If you meant that ele weakness was different for Lghtning, then I suppose that's true. When stacked, Lightning's damage % is reduced by 100 and halved, then stacked additively (so level 20 lightning stacked would give +12.5% damage rather than 1.25x damage), and the elemental bonus for lightning will increase or decrease the damage added. Basically making it so at level 20, stacking lightning with another charge always provides +6.25%, +12.5%, or +18.75% damage depending on the target's weakness to lightning. Lightning on its own is the same as any other element.