Welcome to the jungle young paduan.
For what it's worth: I buy commercial epubs all the time that run afoul of Flightcrew/IDPF's font-extension/mimetype catch-22. At some point, the bottom line is to use what works most consistently on as many platforms as you can afford to physically test on.
The other piece of advice I would give is to just not use embedded fonts unless it was absolutely necessary to ensure that all characters/glyphs needed for your book render properly. In other words: if it's not a question of special characters being required, then let the ereader (or app) choose the font to use based on your suggestions (that include generic fallbacks) rather than embedding fonts and forcing the issue.