Regarding Manuscripts ... how do we know for sure ?

Men wrote as they were moved by the Holy Spirit. They were so Spirit-filled at the time of writing that every word they wrote was in fact from God. German higher criticism began with the wrong premise that only works verifiably written by prophetic or apostolic eye-witnesses should be considered inspired, and it only went downhill from there.

As for "poetic license," sometimes God actually did "dictate" what was to be written, as in some prophetic utterances in the OT, but it's also clear that God directed men to use sources and their own experiences and vocabulary and grammar to convey his mind. This in itself is a miraculous thing, that the infinite God could use finite men (some with poor learning) to communicate his revealed will perfectly. But with God all things are possible.

As for your example in John 21, the difference in the words is likely only a literary technique to avoid exact repetition, which happens elsewhere in John (and even right here with the words for sheep!), and also because they were likely speaking Aramaic and the main point was the 3 times since Peter rejected him 3 times.

As for Greek as the primary language of the NT, we know this from the literary artifacts that have been handed down, from patristic sources, from linguistic analysis of the Greek itself, and so on. That said, it is natural that some Aramaicisms and other linguistic features remain in many NT works due to the writers and their sources that derive from the Palestinian itself. Also, it is possible that a document (e.g. Matthew) was originally written in Aramaic and later interpreted with the wording of Mark when suitable or altered when thought appropriate. In such a scenario, which likely would have occurred at the point of canonization around the turn of the first century, I assume an apostolic (like John) or prophetic Holy Spirit gifted person (like Timothy), or a group of such persons, puting the final touches on the New Covenant. This new covenant (or the canon as we know it) would begin competing with previously circulating independent works on the Christian tradition (some inspired and some not). The canon won out because it had the power of the Holy Spirit behind it.

I hope I answered your questions somewhat.

 

1. I would say that the writers accurately recorded what was said...ALTHOUGH, it is likely that Some of Jesus' teachings were shortened for the Gospel accounts...which doesn't negate accuracy, for example:
-Mom to Son: "Please tell your brother that I would like him to go to the store and get me some milk."
-Son to other brother: "Mom told me to tell you that she wants you to get some milk from the store and bring it back to her."
--> Did the brother accurately portray the mother's message? Yes. I believe this could have happened in the Gospel accounts, but was not a corruption of Jesus' message because their paraphrases themselves were guided by Jesus' Spirit himself.

2. It is my historical understanding that they were (mostly) originally written in Greek (with some arimaic expressions), even though it is likely that much of Jesus' ministry was in Aramaic.

3. Regarding a translation of a translation, here's my answer...If Jesus spoke in Arimaic, and the people who heard him (mark, Matthew, John) or interviewed those who heard him (Luke) wrote the things he said in Greek, with all the necessary adjustments and translations needed, We must remember that the same spirit that guided Jesus as he taught also guided the writers, so it is not the same as if I translate something from Korean. I may make mistakes...but if the Person whose Korean teaching also spoke perfect english, and came and helped me with my translation and made sure I got it right, then It is safe to say that my translation accurately portrayed his teaching. (So any translation after that could be considered a first-level translation...especially if the person who spoke Korean was the perfect God.)