What do the pronouns indicate?

Whatever one believes about the merits of completing the remaining intercity portion of the system, it is clear that 1) it gives the country an intercity transportation network that would be sorely missed had 2) it not been built. Even those who argue against 3) it 4) do so on the ground that if it had not been built, some better (nonauto) system would have been divised.

Does 1) “it” indicate completing the remaining intercity portion of the system?

Does 2) “it” indicate "the remaining intercity portion"?

Does 3) “it” indicate the same as 1)?

Why does 4) “do so” appear in the present tense and not in the past tense?

And what does “do so” indicate here?

Answer

Edit note: HEALTH WARNING & ACTUAL ANSWER

I managed to track down the original text that this extract comes from. It turns out that a bit more context would indeed have made this text very easy to understand. Most importantly, the pronouns 'it' here actually refer to an item mentioned in the previous sentence, not in the given excerpt. In particular, all the Original Poster's 'it's refer to "the interstate highway system". It's a good reminder that a pronoun can be used to refer to something that's been mentioned way back in the discourse. It certainly doesn't need to refer to something in the same sentence! Here's the original - with a tiny bit more context:

Consider the interstate highway system. Whatever one believes about the merits of completing the remaining intracity portion of the system, it is clear that it gives the country an intercity transportation network that would be sorely missed had it not been built. Even those who argue against it do so on the grounds that if it had not been built, some better (nonauto) system would have been divised. Yet most observers would agree that the interstate highway system could not have been built if it had been proposed in the mid-1970s rather than in the mid-1950s.

We can understand the paragraph like this:

Consider the interstate highway system. Whatever one believes about the merits of completing the remaining intracity portion of the system, it is clear that the interstate highway system gives the country an intercity transportation network that would be sorely missed had the interstate highway system not been built. Even those who argue against the interstate highway system do so on the grounds that if the interstate highway system had not been built, some better (nonauto) system would have been divised. Yet most observers would agree that the interstate highway system could not have been built if it had been proposed in the mid-1970s rather than in the mid-1950s.

Do so

The reason that do so is present tense is because like its antecedent, argue against it, it refers to people who currently argue that the interstate highway system was a mistake

Refs: Lester Thurow, 1980, The zero-sum society. New York; Basic Books.

Edit note: I was going to leave my wrong and long answer here for readers. However, I thought this might just waste readers' time in the end. If you want to read it for giggles you can have a look in the edit notes!