Main Question Answer:
Yes that will work and your Druid_Starfire_AreaUnit will be set to s new variable every enumeration. Example Druid_Starfire_Group consists of unit Arthas and unit Lich. At first, lets assume Arthas gets picked first, AreaUnit = Arthas and Caster will damage Arthas if the conditions are true. Then Lich is picked next so AreaUnit is now equal to Lich and Caster will damage Lich if the condition is true. Conclusion is a variable can only hold one unit at a time.
Small Questions Answer:
1. I think it doesn't. But Elune's Grace does.
2. I think it does.
3. It makes the unit with Resistant Skin treated as a Hero, as simple as that.