Can you install the Combi Coccoro with a lap belt?

It depends on the type of belt actually. If you pull the belt all the way out and then it starts locking back so you have to put it all the way in again to pull it out again, then it's safe. If it is one that has a tail that you pull to tighten then you need to make sure that the locking feature in the tightener is not bent and still works, or it may not be safe. Usually it's fine without the shoulder belt, but you need to make sure your lap-belt is approved for the car seats that are around today.