Learn English – Go for/on a trip

According to this Wiktionary page,

A small number of English adjectives take noun phrase complements. CGEL lists only four: due, like, unlike, and worth. Underweight and probably overweight are also in this class. There may be others.

So, "a major cyber attack.." is a complement that the adjective due takes in your sentence.

The meaning, again according to Wiktionary's entry on due, is "owed or owing". Wiktionary provides the following example:

He is due [four weeks of back pay]. (a noun phrase complement in square brackets)

The note "not before a noun" means that due in this sense cannot be used as an attributive adjective, but only as a predicative or postpositive adjective. An attributive adjective is used like this:

The threat of cyber attack should be given due attention. (the adjective due stands before a noun, and this noun is attention)

In your example sentence, due is used predicatively. It would be hard to use it attributively in this sense anyway, since we need to attach the complement to it somehow:

The experts say that our world is a due [a major cyber attack causing widespread harm before 2025] world.

(I used due before the noun world, that is, attributively, and inserted the complement between the adjective and the noun. The result looks outlandish).

Yes, there is a difference. One usually doesn't have a meeting "over" something, without using more words. For instance,

"All the employees at SEI Corporation are invited to the meeting where we will go over the new internal communications system that the company is about to bring in."

You are correct, however, that "over" can also mean "about something," as in this example:

We argued over who would do the dishes.

So why wouldn't we say, "We had a meeting over who would do the dishes"? It is largely idiomatic, but to a native English speaker it would sound odd.

A better choice than either "on" or "over" in the original example would be "about," except that it is used later in the same sentence. The sentence could be rephrased to use "about" like this (changing the second instance to "going"):

"All the employees at SEI Corporation are invited to the meeting about the new internal communications system that the company is going to bring in."