Interesting question.
My opinion would be that the digestion will be slowed, but perhaps not as much as you might like. With whey/milk/PB, it just seems like a shotgun approach with many different protein types. You will get slow absorption from the casein that's already in the milk, but as for the peanut butter slowing the digestion of whey, that's debatable. I know that fats are slow digesting compared to protein and carbs, and I also know that they can cause some encapsulation of the other food you consume with it, thus slowing their digestion as well.
So if you thoroughly mix your PB with the whey, using as small amount of liquid as possible, you could probably cause a slowed digestion. How much, I'm not sure. However, if the liquid is your milk, you're sacrificing the slow absorbing casein that's in it by using such a small amount. I'd go with a casein type protein, which is guaranteed to have the properties that you want at night.
I don't see much of a cost benefit either way, but using a food source like cottage cheese for casein would by recommendation.
Personally, I mix the cottage cheese with tuna and green beans for my last meal.
