If lotion/cream/body oil was applied the night before (or perhaps 8+ hours prior to use), would the area have to be wiped off with alcohol for Ray Gun to be effective?

It is best to still wash the problematic area before applying Ray Gun.

Personally, I would wash the area of the body to be treated so that no other oils or creams are left on the body. Once the treatment has been completed, then you can apply any oils, lotions or creams on the body area.