Learn English – Difference between “validation” and “verification”

Surely there should be more of a difference between the two words. If they mean the same thing surely there would be no need anymore for two words

There are a lot of near synonyms in English, so this does not follow at all. It's particularly so when you consider that act entered the English language in the late Middle English period, from French, while deed was in the language from the very beginnings of Old English. Many, but not all, seeming redundancies in English vocabulary have a similar origin story.

It is true that there are differences in nuances. The primary one would be that an act would generally be need to be relatively remarkable before we would use the term deed. One can do a particularly brave or noble or dastardly or evil deed, but one can't really do a lazy or everyday or common deed. There's nothing technically wrong with the latter, but it wouldn't be an idiomatic use.

There are similarly some differences in what verbs and prepositions are used with either. One doesn't do an act, one cannot be in the deed of anything.

In your examples, 1.a is correct because it is referring to what the subject, or person, is current hearing. it is still in progress. 1.b would mean the same thing as " I can't believe what I am able to hear from here". It does not refer to what you have heard, instead it refers to what is possible to hear.

2.b is more natural and you are correct in that scenario. What you should consider is that since it is a movie, they may have only shown the character witness the dark entity once but still could have seen it after as well.

In example 3, both are correct but differ in meaning. 3.a means that the subject is still seeing it. Also, it could be referring to something specific. With 3.b, it is more open ended so the question could be general, as in nothing specific.

example 4 seemed to have been well answered in the other forums. I hope this was a little helpful.