What If? - Past Tense Sequence

Let's hope no future historian has cause to write such a sentence about any two countries, let alone yours and ours!
Note what parts of the verb construction have to be there, and in what order they come. If we leave aside the meaning, you can take away the 'never'; if you also peel away the passive ('been'), you are left with ' ... would have started', which is perhaps rather more manageable.
We can get rid of the 'been' anyhow: ' ... WW3 would never have started' is fine as a clause, if not as a historical idea. In this case the War itself becomes active, rather than passive ('been started'); we have already considered which country might have been to blame, so that passive is not really necessary.

Either 'if she were watching', or, better, 'if she had been' (but, sadly, she wasn't) ...then her son 'could not have (had)' the accident.
Parts of some of the other Answers are good, but none of the others hangs together as well as this one.

All three of the silly suggestions are, of course, complete fantasy ~ so we need 'were', and then we wonder what result 'would' follow.

All four Answers contain the right words here, but only No.4 is an acceptable order. You may wish to think your way slowly and carefully through this in order to understand its workings in full detail.
The only other possible order (not offered here, to avoid confusion over the selection and scoring!) would be:
' ... I never would have committed ... ' (This brings the 'never' as far forward as possible, to emphasise it forcefully).

Answer 1 is correct here: apart from anything else, it is the only one with the proper past form of 'know' at the end of the second blank.

Answer 2 is also good, but the present-continuous verb form in No.4 picks up on the idea of 'today' in a more immediate way.
Answer 1 is grammatically possible, but rather silly in fact ~ because, clearly, we do actually have computers!

We need a Pluperfect formation here ('past perfect'), because her realising should have happened even longer ago than the weekend (which, itself, is one step into the past from 'now'): so there needs to be a 'had' somewhere, to express this time-distance clearly.
The 'had' can go on the front, as here ~ almost as though in a question ~ to set up an 'if'-type condition, but without using the word 'if' itself. (Turning the verb round like this is considered sufficient indication that we are setting up a hypothetical situation; see also Question 4, above.)

'If one thing were true, another thing would follow.'
Try to remember the basic structure: set up the 'trigger' first, using an 'if'-clause' and (usually) a verb structure including 'were' ... then follow with the consequence, using 'would'. 'Were' comes before 'would' alphabetically (just!); the 'trigger' needs to come before the possible result (though sometimes, for effect, we may state the result first and then explain what might prevent it: 'We'd be in deep trouble if we were on the wrong side of that line'.)
It is a sad thought that some people in our world are not only hungry, but they have no bed to sleep in; and even if they did, they may still be too hungry to sleep at all. Let's hope that your study of another language may help to make the world just one tiny, but useful, bit smaller ...

'If I were ... , I would ... ' is the usual formula for such an imaginary situation. It all sounds a bit as though it is in past tenses ~ but this is just a convention of language, to bring the situation one step further away from actual reality.
Answers 1 and 2 are too definite; No.4 is not definite enough!

'If one thing did happen, another thing would not' (like heads and tails on the toss of a coin).