The dialog is a GTK FileChooser, and as such it can be customized via the gtkrc file in the theme (but since it it the obsolete GTK2, documentaiton is hard to find), but A, B, E, F would be handled there.
[D] is already there: Preferences ➤ Image import & Export ➤ Export file type ➤ Default export file type
No idea for [C], but I see plenty of good reasons to not do it...
[D] is already there: Preferences ➤ Image import & Export ➤ Export file type ➤ Default export file type
No idea for [C], but I see plenty of good reasons to not do it...