The MOBI format does not support embedded fonts. Only the device's or application's own fonts are used. If the needed glyph is not available in those fonts then you're out of luck.
A properly built PDF (very small margins, proper text size relative to the chosen page size, ...) is the only real option, but then only if you are building a personal document. You loose reflow and user choice of text size so you must build the document for a specific target device so its not practical for general distribution where the file will be used on devices with a wide range of display sizes.