i just wonder, those 4 elemental feats, aegis of rime, aqua mortis and so on... do they increase damage only from spells, or is it also from weapons and items like flaming oil...?

They do increase elemental damage from weapons, although usually not by much (20% of 1d6, for example, means you get one extra point if you roll 5 or 6). I'm not sure about flaming oil and the like.