I can't believe this has gone on for 5 pages. The simple answer is "yes", passengers can have a go, obviously you should trust them, they should be briefed properly and only given control when you're not busy etc, but its a yes.
People can say that this may cause accidents but thats ridiculous because if you're going to be like that loads of things cause accidents, like crossing the road without a zebra crossing but there's no law against it and there shouldn't be. The chances of a passenger on the controls causing an accident are absolutely minimal and if you don't want to take the chance then don't let your passengers take control, but there shouldn't be a law against it.