1. The span is total cruft, probably left behind by some messy word processor auto-html. Feel free to get rid of it. Spans are only ever important in an e-book if they have a class as a hook for some sort of CSS formatting or a lang attribute to denote a foreign language phrase you might want to format differently to make it stand out.
2. That particular div looks superfluous, especially with a height of 0 em and nothing inside. If a div has nothing inside, assume it's cruft and delete.
Only if it's got a height set might you possibly want to keep it in case it's some sort of important scene spacer, but you can get a much better effect by wrapping a new div around the entire "spaced" section and giving it a class hook to anchor some CSS formatting to get the linespacing effect (or just put in a manual spacer like an <hr /> line).
You wouldn't need a <p> with height set to 0, because the default for Mobi display on e-ink Kindles (may be different on K4apps and other readers) is for no spacing between lines by default anyway. However, you could wrap the <a> within a <p> and omit the <br /> as you'd get a line-break with the <p> anyway and that would allow you to style the formatting later in case you decide you do want some space between lines or you want no-indent in the TOC entries (the functional part of the hypothetical <p width="0">).
3. That <a> is actually functional and pretty important, as it acts as a link to wherever Chapter 3 is located. You need to keep that one. But you can get rid of the font tags. You can't even see colours in most Mobi-supporting devices anyway (maybe K4PC/iDevice apps).
All you really need to look at in an HTML tutorial is what the "presentational attributes" possible for a given HTML tag are, because that's all you'll be able to use with Mobi, and you'll only be able to use a small subset of them (eg. anything involving setting a colour, you can ignore).
Hope this helps.