Kashrut

Question

1. Why does everyon say to double wrap things when you put them in an oven used to cook non-kosher things? Why isn' wrapping it once enough? 2. If it is a shaat ha-dechak, can you wait less then 6 hours after milk before you eat meat? For example, let's say you forgot that you were going to someone's house, and you had a piece of meat, and then you go to the house and they are serving dairy. After they went to all the trouble to make you a meal, are you supposed to say sorry, I can't eat it? That would be very hurtful to them.