The use of "the" and plural concerning "case"

Q1

For the first, the question is about the use of the particle "the" in the construction "(the) nominative case". Both "… use nominative case" and "… use the nominative case" are frequently found. So does the make a difference here? or is it optional?

I have a guess here. "the nominative case" is about a declined form in a certain language, while "nominative case" is a general, abstract, translinguistical concept, as well as the difference between "I like the dark chocolate" and "I like dark chocolate". But it failed to explain the use of "the" in a sentence like "In grammar, the nominative case […] is one of the grammatical cases which…". Apparently this sentence is not about a certain form in a certain language, but about a general, abstract, translinguistical concept, so my guess might be unreliable.

Q2

Another question is, should I decline the word "case" in singular or plural in the following sentences. Here are some examples of the singular form I found from Google:

Today I would like to give you a first impression of the nominative
and accusative case.

let’s quickly review nominative and accusative case.

Intuitively the singular form sounds more correct for me, but according to grammar it is wrong, since the nominative case is one case and the accusative case is another case, so they are two cases together.

The plural form is frequently found on Google as well:

For example, the nominative and accusative cases often distinguish
subject and object of the verb…

Understanding how the nominative and accusative cases work in
German…

But they sound unfamiliar to me. So, which is correct?