You cannot change the default font in the mail app. You can select text and set it to be bold, italics or underlined or any combination of the three - but you are stuck with Helvetica as the font.
If you want to change the font in the Notes App - Settings>Notes>Font - there are three choices.
I know nothing about iNote, but if the capability to change the font is not built it, you are also stuck with whatever the default font is.
I am quite sure that you cannot change the default font in the mail app. Apple has not allowed for it in the settings. There may be a third party email app that includes fonts that allow for choices, but not the built in mail app.
That list of fonts that you found includes all fonts that can be used on the iPad, if you have the apps that include fonts within them. For instance, the Pages App by Apple has a number of built in fonts that you can select from - but they can only be used within that app. Other word processing apps may include other fonts as well. But you can only access other fonts if they are included within the app - for use within the app - and can not be used elswhere on the iPad.
There is no way - at least that I have ever seen - to change from the default apps on any of the preinstalled Apple apps - except for the Notes App. Even there - you are limited to those three choices.