Sentence structure

I have several questions regarding a sentence in the paragraph below.

The halo effect causes one trait about a person to color your attitude and perceptions of all her other traits. Even stranger, the more noticeable the aspect is when you form your first impression, the more difficult it becomes to change your attitude about that aspect. So, for example, if you are bowled over by the warmth and kindness of a coworker in your first week at a new job, you’ll let him get away with a host of obnoxious behaviors later on, maybe even for years. If the first year of a relationship is deeply fulfilling and life-altering, it can take a long time to notice if things turn sour later. If you like specific aspects of an individual, the halo effect causes the positive appraisal to spread to other measurements and to resist attack.

Q#1. What does 'it' refer to in the underlined sentence above? [to notice if things turn sour later] or [if things turn sour later] ?

Q#2. Is the clause 'if things turn sour later' nominal or adverbial?

Q#3. Practical English Usage says that it is used to introduce some clauses with if, as if, and as though. The example sentence is, "It will be a pity if we have to ask her to leave." Is it possible 'it' refers to 'if things turn sour later' in the paragraph?