I'd go with this.
Ghostly Forms: Multipower, 60-point reserve
6f-Incorporeal Form: Desolidification , Reduced Endurance (0 END; +1/2) (60 Active Points)
2f-Semi-solid Form: Physical Damage Reduction, Resistant, 50% (30 Active Points), Costs END only to Activate
2f-Semi-Solid form: Energy Damage Reduction, Resistant, 50% (30 Active Points), Costs END only to Activate
Flight 15m, x4 Noncombat, Costs Endurance Only To Activate (+1/4) (25 Active Points); Limited Power Power loses about a fourth of its effectiveness (Only when in Incorporeal form; -1/4)
This way the character can interact with the regular world but is Desolid as a default. The Semi-Solid forms should be considered a Combined Power (Sorry, I had already closed HD).