1. That, and also barrier conditions reduce stats. For example, Poison reduces yang def (and attack), so poison pitfall actually reduces yang def 5 levels: 3 from the skill, 2 from the poison. For the other effects, Burn lowers yin atk/def, Freeze lowers agility (lol), Paralyze lowers evasion, and Blind lowers accuracy.
2. In general, yes. For Marisa, no. Ran can help do damage, but lacking wood bullets hurts her.
3. Medicine. it doesn't matter if her attacks aren't strong. Her utility comes from being able to easily perform full breaks on Rojo and the fairies. 3* Medicine can consistently solo Rojo over three turns
4. Yes, but only if you've hit max Yin/Yang atk up and enough accuracy to always hit. Since you want to do it in one turn, stick to attack or accuracy.
5. See 4. I don't know how much def-scaling adds to damage, but Reimu already has many ways to increase her defenses.
Are you aiming to do the one turn manually or full-auto? If it's the former, posting what characters you have available would be smart. If it's the latter, I wouldn't bother. I haven't heard of comps that work that don't involve Kaguya or Yuyuko.