I be very handy to have character sets switching tool like FF has.

“View->Encoding->…” and here menu which shows all available character sets

Doing choice in this menu will change configuration parameter “default_encoding”.
Also it will be usefull to allow users to setup encoding by shotname: cp1251 or koi8-r. It is really impossible to findout full name for encoding: Cyrillic (Windows 1251). I spend too many time to find that (((

Also, to my mind, fallback_encoding and default_encoding seems are same.
see your description:
default_encoding: Encoding used for files opened with an undefined encoding
fallback_encoding: encoding to use when the encoding can’t be determined automatically in other words undefined

So you have two options for one thing. Think about to remove fallback_encoding, because of it is useless