Decorator Pattern Questions

Recently I read about the decorator design pattern but I was left with some unanswered questions that I couldn't find online. I'm not going to show code because I don't want to make this question more complicated than it really is. I'll simply give an example:

Subway Store:

Component --> SubSandwich

ConcreteComponent --> 15cmSub, 30cmSub

Decorator --> Ingredients

ConcreteDecorator --> White Cheese, Yellow Cheese, Jam, Chicken.

This is just the way a Subway store works. Pick your core sandwhich size and then add all the toppings you like. But I still have some questions:

1. What if there is an invalid combination of ingredients? For example, Subway policy says that there can't be two kinds of cheeses in the same sub. Now let's imagine that there are 10000 possible combinations for the toppings and ONLY one is invalid. Does this completely break the decorator pattern?

2. What if two ingredients are dependant. For example, if you order lettuce THEN you need some other kind of vegetable to make a "valid" Sub.

3. When is it better to use the decorator pattern instead of a SubSandwich class with an ArrayList of Ingredient? I know here the Ingredients don't add behaviour, which makes the Subway example not accurate, but let's assume they do.

4. Why extending? Why not using interfaces?