I hope I will help you here; if I confuse you in any way, let me know. I am not good with the grammar of English so I apologize for not being able to explain the rules of grammar in your third question. I am looking at the phrases from the viewpoint of creative expression because that's my strength, not grammar

1) An example of a person getting a 'shiver' is if he goes outside on a cold day, the sun is shining brightly, so he does not put on a heavy coat: he walks outside, the wind blows and he shivers from the cold air he feels: a shiver is not voluntary, a person has no control over it, it comes, then goes quickly. I think in the story the freezing air brings a shiver but fear also 'incites' the response.

To incite means to bring about an action or reaction from a person. A person feels the urge to take action: for example, people can be incited to vote for a candidate by a encouraging speech or, to the extreme, a group of people can be incited to riot by another kind of speech. Here, I think, the freezing air brings an involuntary shiver to his body and you are right: because he does not know what exists within the 'emptiness', his fear brings about an involuntary shiver: he feels fear because he anticipates something in the emptiness but he does not know what it will be and that 'incites' a fear he can't control

2) I think 'blinking into the mirror' is a creative way to say he keeps looking into the mirror, he hopes flattening his hair down without a comb will be successful, he hopes to 'blink' and see his hair flattened down but he knows that is not possible: it is like if I go to an ice cream shop where I always get chocolate but one day chocolate is not on the menu, I may look at that menu 2-3 more times, hoping I did not read the menu correctly but I know what I read is correct: there is no chocolate today!

He is 'blinking into the mirror' because he has to look into the mirror to see his hair to flatten it down but he wants to avoid the sight of his messy hair. He hopes that flattening it down with his hands will work but when he looks in the mirror again, his hair has not changed. I don't know the story, is he was a person who always had a neat appearance? If that is true, when he sees his messy black hair, something that he can not change at the moment, it is like looking at another person, a person he does not recognize as himself: if he were to describe himself to others, the image he looks intently at, or stares at, in the mirror would not be the image he would include in his description.

He cannot ignore what he sees in the mirror, he is confronted with the sight of this 'someone' with messy hair. He sees this image in the mirror, not with blinking eyes that can blur his vision, but with "eyes wide open" as the saying goes - and all he can do is stare back at the image

"Eh, it's close enough": he cannot change what he sees in the mirror, so he accepts that. He is only giving up trying to change his hair at this point in time, he knows he will have to accept this messy black hair at this time. He knows standing in front of the mirror won't change his hair.

He then tries to become less serious about it because he needs to leave the mirror and live his life; "eh" is an expression meaning "What can I do? My hair is what it looks like." An example may be if I want to dress a young child in matching colors (say blue and white) but she wants to wear clothing that have five different colors. I try to get her to change into the clothes I want her to wear but she is stubborn. So I say "Eh" as I lift up my hands in a gesture saying "I give up, you win!" and I let her go out to play in her choice of clothing because there is no point in trying to change her mind and this is not something important to cause an argument. Likewise, he is saying "eh." "I am done trying to make my hair look like it will not, it is time to 'move on' more important things."

His saying "it's close enough" I think is just a bit of humor to lift his mood and stop focusing on his messy black hair: an example may be if I want to wear a blue jacket to go with my jeans but the only blue jacket I have available is a purple-blue shade; my sister says "that's not blue" and I say "it's close enough!" because I need to leave the house to go to work. This is like saying "this will have to work for now, let us (or me) stop thinking about it:" whether we are talking about his messy black hair or my (not blue) jacket.

3) He can't "be bothered to scale all the way to the roof" means climbing up to the roof, 'scaling' would say to me he would have to climb a ladder (maybe stairs but I think ladder it is) to get to the roof, because to scale also can mean climb.

"if for whatever reason:" you are right about it meaning 'in case.' To me, it has a tone of frustration also: an example might be when my father went to the bank at 13:00 pm, he would say "I will be back in 15 minutes. If for whatever reason, the line at the bank is long, I'll be back in 30 minutes:" I heard him complain many times about long lines of people at the bank after lunch time, he would say in frustration "why were all these people there at the bank, lunch is over, don't any of them work?" because he wanted to come and go to the bank quickly, without any delays. He did not have to say that every time: he only had to say "for whatever reason" and I knew he meant "I can't think of one good reason why there should be long lines of people at the bank." The character can't think of any good reason a crowd of people would be on the roof but it doesn't matter what he thinks: there probably is a crowd of people on the roof.

Again, I hope something in here helped, Mary