> **Question 1**: How often are offers made before the end of the interview period (i.e., before all candidates have been interviewed)?

I would say "early offers" only occur in exceptional circumstances. Universities have hiring policies and those policies require dotting i's and crossing t's -- paperwork and process -- particularly for tenured or tenure-track positions.

> **Question 2**: If a candidate expresses that a school is her top choice, does it make any difference to the hiring committee?

Let me put it this way: you do **not** want to express that the school is not your top choice. The hiring committee are looking for a monogamous relationship. Talking about all the other schools you're going on first dates with isn't a great idea.

> In fact, can expressing this kind of preference actually hurt a candidate's chances? E.g., perhaps it makes this candidate look like a "sure thing," which frees the department to first make an offer to another, "higher-risk" candidate. In general, when is a good time to express such a preference? 

This makes absolutely no sense. 

> **Question 3**: To what extent do different departments talk to each-other?

I guess you refer to the US? If so, I don't know but I would imagine there's very little communication on an inter-department level (through people in different departments may informally communicate). 

> For instance, is there any mechanism in place to prevent candidates from "falling through the cracks?" E.g., one can easily imagine a situation in which lower-ranked schools don't make a candidate an offer because they expect she will get an offer from a higher-ranked school; subsequently, the higher-ranked schools make her no offer and she is left without a job. Likewise, if I express a preference for school X before visiting school Y, do I risk pissing off my hosts at Y?

This again makes no sense. A university is unlikely to avoid offering you a position because you might be "too good for them" or that you might get other offers (in the worst case, it only costs a university a couple of weeks to make an offer for a position that will last many years). 

A university may avoid offering you a position if you seem disinterested or if it seemed likely that you were using this position as a short-term stepping-stone to elsewhere. 

Different departments are not going to compare notes. They are competing with each other.

> **Question 4**: Do I just need to relax?

Yep. Maybe ease up on the coffee and take up croquet or smoking or something else instead.