This is a sentence from a site discussing free modifiers:

It is not of the games children play in the evening that I want to speak now, it is of a contemporaneous atmosphere that has little to do with them: that of the fathers and families, each in his own space of lawn, his shirt fishlike pale in the unnatural light and his face nearly anonymous, hosing their lawns.
                                                                                                    —James Agee

Question 1: Does ‘not’ make the first clause not a main clause, thus avoiding a comma splice? Is there a name for this clause/structure?

Question 2: ‘that of’ after the colon. Is it continuing on from ‘it is (that) of a contemporaneous atmosphere/it is that of the fathers and families…?

Question 3: I see every phrase after ‘families’ as absolute phrases, followed by one pariticple phrase ‘hosing their lawns.’ Do you agree?

Question 4: Why can the preposition ‘of’ be omitted here, “that I want to speak (of) now”?

Thanks.

It is not of the games children play in the evening that I want to speak now, it is of a contemporaneous atmosphere that has little to do with them: that of the fathers and families, each in his own space of lawn, his shirt fishlike pale in the unnatural light and his face nearly anonymous, hosing their lawns.

                                                                                                   
This sentence is actually full of mistakes, as touching as it is.

First, the sentence contains a comma slice, with or without the not.

Is there a name for this clause/structure?

Yes. It’s called an error.

‘that of’ after the colon. Is it continuing on from ‘it is (that) of a contemporaneous atmosphere/it is that of the fathers and families…?

I would say it’s “that [atmosphere] of the fathers…..”

I see every phrase after ‘families’ as absolute phrases, followed by one participle phrase ‘hosing their lawns.’ Do you agree?

Yes. But since he says fathers and families, the rest of the sentence should be, “all with their own space of lawn, their shirts fishlike….and their faces….”

Why can the preposition ‘of’ be omitted here, “that I want to speak (of) now”

Cheers.

At first glance I assumed it was a comma splice, but I thought this writer wouldn’t make this error, so I thought the ‘not’ may have something to do with making it dependent on the following main clause…

What about this one then:

World war 1 began like a summer festival—all billowing skirts and golden epaulets

                                                                —Dalton Trumbo

The explanation states that the phrase in italics modifies ‘summer festival.’ But I thought that ‘all’ was a pronoun here. therefore, billowing…modifies the pronoun…

What do you see this phrase as, please? If it is in fact a pronoun, then this would make the phrase an absolute phrase. therefore, the phrase doesn’t modify summer festival as the site explains.

World war 1 began like a summer festival—all billowing skirts and golden epaulets

This one is difficult to analyze because the phrase is a metaphor and not literal in meaning.

However, I would say that all billowing skirts and golden epaulets redefines festival and is therefore an appositive. The festival, in the eye of the author, was billowing skirts and epaulets because that’s what he saw.

O.k. thanks.

So is all a pronoun renaming festival, and the rest of the phrase modifies the pronoun?

In this sentene, all is an adjective modifying skirts and epaulets.

The coat is all wool. [adjective]
That is all. [pronoun]