Is this news script grammatically correct?

As others noted, "p.m." is an abbreviation, and it's all just one sentence. But let me add: that sentence is poorly written and not a good model for learning. We refer to this as "purple prose," when the style of writing is so excessive that it distracts from the message. Contextually, it's understandable, because it's newspaper writers working on a tight deadline on a genuinely tragic situation. Nonetheless, it's poor writing.

The sentence structure is a little odd, but it is grammatically correct. The part you highlighted is elaborating on the subject noun of the sentence. You seem to be confusing the last period in the acronym p.m. as the end of a sentence, though, not misunderstanding how English sentence structure works.

If you take out the fluff and some of the detail, you have: That flash erupted, a collision claiming lives, triggering human responses. I would use two-or-three sentences to say what they did. To use one sentence I would add a few words to clarify and add symmetry in the verbs. “The flash that erupted was a midair collision that claimed lives and triggered the gamut of traumatic emotions.”

I believe you are confused by the period in p.m. and the capitalized Wednesday. The sentence continues from earlier, the article's first sentence does not end with p.m.

The sentence doesn’t start with “Wednesday”, it starts with “That” at the beginning of the paragraph. There is a period after “9 p.m.” for stylistic reasons, but not because the sentence has ended.

The sentence starts with 'That'. 9 p.m. Wednesday is a single descriptor of the time the accident took place. The main verb of the sentence is 'erupted'.

I studied journalism, and this is what you call a lede. It's a one-sentence paragraph that summarizes the entire news article.

No. To should be two. It also contains many non-sentences.

Aside from the misunderstanding about the period, the sentence is a run-on. Simply too much going on to easily keep track of and understand.

Grammatically correct, yes, but it’s bad writing, especially for a newspaper that’s supposed to be clear and direct in its communication.

Unfortunately, since this is the kind of material most available to learners, newspapers and related media are a very poor model for learners. They heavily rely on conventions and cliches developed in order to save space in newsprint. These conventions are now deeply embedded in journalist culture and are retained even in online spaces where the need for them no longer exists.

Grammatically, it appears to be correct. But, like most, I had to read a couple of times to fully understand its meaning. Clarity is its main problem here.

It's terribly written whatever it is. I am a native speaker and had to re-read the extraordinarily long first sentence several times to understand it.

The sentences starts with “that”. This is a correct sentence

There is definitely no verb. There are many ways this could be fixed, but the easiest would probably be to put the two verbs in past tense. It could be done leaving triggering and removing the 'and' before it.