All the various ebook reader software and devices like to do their own thing with fonts and override your embedded fonts. Most devices (Kindle, nook, Sony) give people the ability to choose their font and size (from a few choices on a given device). In other words, you really cannot control how people are going to see your ebook. It's going to look different, depending on the person and device. I've found it's easiest to “let go” of the font and focus on other formatting issues.

Make sure that the TOC style is in the document that is defined as the style source in your book file (with the little icon next to it). Also when you export, be sure to check suppress automatic TOC entries.