Lots of questions. Here we go:
1) Yes, "it" refers to the completion.
2) Yes, you are correct again.
3) Yes. In fact - 1, 2 & 3 are all essentially the same thing. Which is why they have used "it" instead of changing the subject.
4) It is in the present tense because the whole paragraph is in the present tense. To be in the past tense we would have to say: "Even those who argued about it, did so because..." which would suggest this argument has been completed and is over.
However, the argument still exists and they are still arguing their opinion, so the present tense "argue" and "do so" are correct.
Do so = that argue.
I hope this helps.
Tom