If you're using stationary, the font in the stationary will override the
one set elsewhere. You may have to adjust the stationary to use
the font you want.
Yes. Check the source code of the stationery. Sometimes its set in the
style section of the <BODY> tag and that dictates the font for the entire
page. You can go to View | Source Edit with a new message, and then check
the Source tab to change the HTML, and then you can save the unsent message
as a template on the Desktop to use whenever you start to compose.