Do you mean, why the use of past tense? Well, what you have is basically a second conditional in a question form. The confusing bit here is that past tense (a real action) and past subjunctive (ie. just an idea) look exactly the same. We can tell the difference because real actions in the past are connected to a time. We know this is a subjunctive form also because of "if". Your example of opening the window is not correct. The natural options are: "Do you mind if I open the window?" "Would you mind if I opened the window?" Note that the tenses are consistent.

It's the second conditional. Putting smell in the past makes it more polite as asking to smell her her is very personal. Would goes with a past tense. Will with the present tense. Will you mind if I smell your hair? Will I need an umbrella if it rains? It is probable it will rain. Would I need an umbrella if it rained? It is not likely to rain.

He's asking if he can smell her hair. I'm pretty sure that asking something like that would get you a restraining order.

I have no idea why it becomes "smelled" instead of "smell" maybe he wondered how her hair smelled by touched it with his nose as if he wanted to kiss her hair before he asked that question to that woman. That man asked that question to tease her and wanted to know her reaction.