Learn English – the grammatical explanation of the phrase “all that” as in the sentence “I didn’t understand all that much”

Your question is somewhat misleading, and may contain a false assumption. A statement like the following:

Customers who have never paid a dime

is not a sentence. It's a noun phrase. Let's break that down:

Customers is a noun. Nouns are not inherently subjects or objects, so it's not correct to refer to this as either the subject or the object.

The remaining part who have never paid a dime is a relative clause, which is an embedded sentence that is used to modify something in the larger context. The subject of the embedded clause is the relative pronoun who, and the remainder have never paid a dime is the predicate of the relative clause.

However, using terms such as "predicate" and "relative clause" may be intimidating to people using your software. My best offering for newbie-friendly terminology would be:

• Call Customers a noun (most people at least know what that means)
• Call who a "question word", and make it clear by example what you're referring to
• Call have never paid a dime a "description"

What is there to eat?

The subject is clearly what.

Fairly recently, some linguists have chosen to call there the subject in simple existential sentences, like this:

There is one person in the room.

But this causes problems, as in your example. The main reasons why they seem to want to label there the subject are that it is the first word in a simple existential sentence, and that, colloquially, singular is can be used even when there are several things in the room:

There's too many people in the room.

Another problem is that it results in a completely different analysis of two extremely similar sentences, both in construction and in meaning:

One person is in the room.

Here everyone agrees that one person is the subject.

There is one person in the room.

This is exactly the same sentence, both in meaning and in syntax, except that the adverb there was added, which comes in the first position and causes subject-verb inversion, just as in (some) other Germanic languages.

(In English, this only happens with some words, like there, but in German and Dutch it happens with any non-subject in first position: Ich bin heute in Berlin (normal order) → Heute bin ich in Berlin. (inverted because heute is in first position).)

Why should one person suddenly no longer be the subject? Why should the verb be keep its exact same meaning but suddenly acquire completely different arguments? Occams Razor says no.

In this room is one person today.

By the same logic of those linguists, the first constituent in this sentence should be the subject, just like there above; after all, it comes first, before the verb. It would be confusing to analyse this sentence differently from there is one person today, since they clearly have the same structure. But noöne would suggest this: everyone agrees that in this room is adverbial here and one person the subject.

Further, it seems strange to call a non-noun like there a subject.

Lastly, in languages where adverbs and subjects are more clearly marked as such, by affixes and endings, there is never considered the subject in similar sentences. Doing so in English would cause a rift between the analyses of the same construction in different languages. Both Occam's Razor and those who are learning linguistics would be displeased at such inefficiency. The same applies to comparing the use of the same construction in older English and modern English.

The simplest analysis is to say that there is always an adverb in existential sentences, but that in some such sentences the verb can become singular colloquially. There are many other exceptions where verbs seem to have a different number from their subjects, so this shouldn't be too shocking.

Invasion and recolonisation is not a practical approach.

The police were unable to stop them.

Who is at the door?

Who is clearly the subject here. Replace it with a personal pronoun and it is plain to see:

He is at the door.

He can only be subject (complement).