Learn English – Are these words indeed uncountable

This is the way I think of it and how I always show my students:

A pizza, some pizzas and some pizza:

A pizza is one whole pizza.

Some pizzas are two or more whole pizzas.

Some pizza is just a slice or more of the pizza. It is however much is less than the whole pizza (i.e., less than eight slices).

I usually draw three pictures: one whole pizza, two whole pizzas and a slice of pizza removed (but still near) the whole pizza (don't forget to draw the slice's negative space).

This example also works for cakes, soda (a full bottle, two bottles, some leftover in a bottle), and of course, desserts :).

Good luck.

If you're using demonstrative adjectives with a list, you should use one for each item in the list. See more on "demonstrative adjectives" on this page.

This milk, this butter, and this bread are beyond their sell-by dates.

Even though those nouns are uncountable, you use "this" and not "these," because uncountable nouns are generally treated as singular. See here for more information on that subject.

In your second example, one item is singular (uncountable and treated as singular) and one is plural, so you would use different demonstrative adjectives for each:

This evidence and these witnesses prove him guilty.

Using a demonstrative adjective is only necessary to point to which objects you're talking about. In most cases, you could avoid this problem by simply using an article like "the."

The milk, butter, and bread are beyond their sell-by dates. The evidence and witnesses prove him guilty.