1. As with all Raid Bosses, hit them as soft as you can and as often as you can. Since a single atack counts towards the daily, even though it says "defeat". The reason for the soft attacks bein that the participation reward is the same no matter how much dmg you dealt and the kill reward gets handed out to everyone that participated, not just the killer.

2. Totally RNG

3. Save them until you feel comfortable using them. You could also upgrade your storage capactiy for the girls, if you want to hoard manyuu's and dragons for an event girl, like some people did for Lavender/Edelweiss. I did spend money in the beginning for 2 extra 11 Rolls, but that is not needed. 3* Stars can go a long way, with equipment and high skill level.

4. I don't really use them on lvl 40+ girls since they are a waste of money for the exp they give. I sometimes use them to edge out a level with a high one or use them on frshly evolved ones, since they are back at 1.

5. Yes Lavender was the first Event girl, she might come back in a revival gacha of sorts in the future. Some other Nutaku games did that.

6. Push to Chapter 11 onwards as soon as you can and not spending your time trying to 3 Medal every Dragon Mission you don't need and every mission before Chapter 11. :S

PS: Glad i could help.