I tried your epub on my PRS650. Both headings are displayed in Fontin and the main body text is displayed in the Sony default serif font. See attached screencap.
I'm not sure what your goal is, but if you're using PRSPlus you wouldn't normally need to fully embed any fonts. Usually you'd
- create a /fonts directory on your PRS350 and copy in all the font files you might want to use
- create one or more css files which reference the fonts in that directory. You then select one of them via PRS+ Settings - Book Viewer - User EPUB Style (CSS File)
All epubs will then use that css file until you select a different css file.
If you are going to fully embed the fonts inside the epub then you should probably choose the 'Disabled' option from the above PRSPlus menu.