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).
If you write
One of the main reasons for this is due to the complex orthography of English.
You are basically saying the same thing twice, roughly "this is because".
So either you write
[One of the reasons is] the complex orthography.
or
[This is due to] the complex orthography.
Now you want to explicitly say that the orthography is but one reason, so you have to "tone down" the due to by some restricting expression, like
[This is partly due to]...
[This is mainly due to]...
....
Overlap with your edit:
Yes, your sugestion works too.
Best Answer
It's really just a matter of context (i.e. - the intended meaning).
If the weather condition being spoken is in the future, but the forecast says it will be inclement when it does arrive (so you're cancelling the village fete, or whatever), then of course it's expected.
But if the predicted weather has already started, it's perfectly grammatical to say it's expectedly inclement, if in fact that's what it is (in line with prior expectations). OED defines...
Note that as J.R. says, expectedly is very rare compared to unexpectedly. We usually use as expected when that's the syntactic functionality we want (effectively, He appeared as expected is adverbial usage).
But if we reverse the expectations (and use simpler meteorological terminology), I have no problem with...
It should be fairly obvious that in #1 we weren't expecting bad weather at all (maybe the forecast said it would be sunny). Whereas in #2 we knew the weather would be bad (but we probably hadn't expected it to be so bad we'd have to cancel the fete). Unexpectedly, it was even worse than we had anticipated.
I must admit I don't know why we prefer as expected over expectedly in nearly all cases. It may be connected to the fact that we have many alternatives (naturally, obviously, predictably, typically, clearly, foreseeably, logically, etc. plus phrases like it goes without saying).
Such words are often used in sensitive/loaded contexts, so it's likely people learn to be extra careful with them. Perhaps expectedly is avoided because it raises the question of expected by whom? in too many contexts where we'd rather be more circumspect. So we tend to opt for more "impersonal" alternatives.