It seems to me your online sources are a bit too strict as well as occasionally actually wrong. Qué and cuál do have different uses, but they're not always mutually exclusive.
When there are explicit alternatives to choose from, it is common to use cuál (or cuáles). For example, suppose you're asking a girl to choose between two dresses, one red, the other green:
¿Cuál es más bonito, el rojo o el verde?
But you can use qué to ask about alternatives if there is no previous context, as could be the case in your first example:
¿Qué es más caro, la carne o el pescado?
There's a slight complication here, though: with qué you need not take the gender and number of the alternatives into account, while you do if you use cuál(es).
About cuál + noun, as in your second example, it is perfectly grammatical. As above, it sounds more correct if there are explicit alternatives. For example, you're standing before a shop window and you ask your little girl:
¿Cuál vestido quieres?
If there is no previous context, for example, you're talking about dresses but not about particular colors or models, you can use qué instead, although in this case it would sound more natural to ask something like
¿Qué clase de vestido quieres?
With or without that clase de, what this question means is "what would a dress you like be like?", that is, as you said, a definition.
You use cuál when you ask about a definite item, even if you do not provide explicit alternatives, as in for example these typical phrases:
¿Cuál es tu película favorita?
¿Cuál es tu color preferido?
¿Cuál es la comida que más te gusta?
You can however turn these around and ask (a bit less naturally):
¿Qué película es tu favorita?
¿Qué color es tu preferido?
¿Qué comida es la que más te gusta?
As for your last example,
¿Qué prefieres, la carne o el pescado?
This is basically the same situation as in ¿Qué/Cuál vestido quieres?: you should use qué if there's no context (for example, you're simply discussing food tastes and preferences with a friend), and cuál if there's an actual opportunity to choose (you're holding some meat in one plate and some fish in another one and you need to your friend to decide which one she wants to eat).
I would say you don't need to study a lot of rules, but pay attention to fixed phrases and typical situations when you use one word or the other. As you see, qué and cuál are often both right and their meaning is also very similar. There are only a few situations (as in ¿Cuál es tu color preferido?, etc.) when you must absolutely use one and not the other.
