"From what I hear I reckon I will need another 15 coats of oil to hide the refraction of light. Should I be wet sanding as well to level ???"
I would not think so. If the surface is flat (i.e. cannot feel an edge to the decal) then it is buried. More coats may be needed to visibly hide the decal outline. In reality I'm not sure you ever really completely hide the reflection off the decal surface - in some light mine are always visible!
More coats should help hide the decal edges though.