1. Yes, this is % of triggering skill.
2. Valkiries has own type of attack. This is shown after skill name.
3. There are many factors that affects dmg: critical, balance, rng, defence, etc. So situation you're described not suprising.
You can check element rating of valkiries at the top-right corner of the map. For now all cards has rating 50.
Afaik, there is no elemental strenghts like that, but some bosses immune to certain type of attack and/or element.
4. I don't get this question.
NutakuDev that "initiative" can be misleading. Maybe rename it?
We'll need eng wiki for game. There can be more differences beside droplist of bosses.