1. He doesn't need to wait if he knows the Ramseys are out at that party. He knows exactly when they are back. Remember, he needs to strike when the Ramsey's are asleep. Hell if he wants to he can stake out the party instead of the Ramsey house. That would actually make more sense, IMHO. Or he could actually BE at the party. Would give him all the opportunity to know exactly when the Ramseys arrive.
2. The only reason he needs to fear being noticed watching the house is if he lives in the neighborhood or is known by the Ramseys.
3. If the killer had a family that he needed to account for, he still has to explain why he is not in bed from 11:00pm to 4:00 am. You also have to calculate that there need to be prep time before and after the crime. Especially if kidnapping was the motive.
3. You know, it occurs to me...where would you need to be to stake out the Ramsey house? Presumably you would need to be facing a direction where their bedroom light can be visible.