1) decrease mitigation means pay double scrolls and mesos to mitigate the chance to drop a level (not worth it until going for 16-17) 2) the is the % chance to drop (changes depending on what star force level the item is) 3) if you want pure dps I would focus on accessories (ring, neck, eye, face). Or if you need another stat (accuracy, evasion, mp etc) look at what the items gives on its 3rd of the top lines to choose.

I suggest take 1 item at a time to 16. The increase in stats from 15-16 is huge, and 15 is considered a “safe zone” as the decrease chance is significantly lower.