The difference between ‘Default text font’ under Engrave > Font Styles and ‘Default text’ under Engrave > Paragraph Styles is certainly not obvious, but it is necessary.
Font styles are typically used for things that require font characters – e.g. symbols like clefs, noteheads, time signature digits, etc. – and things that are not strictly text items but which are drawn using text – such as dynamics, tempos, lyrics, and so on.
Paragraph styles are typically used for Shift+X and Shift+Alt+X text items, and for text in text frames, and for a handful of things that are not text items or frames but which require rich text, such as staff labels.
‘Default text font’ therefore affects the default font used by things like tempos, dynamics, lyrics, and so on, whereas ‘Default text’ affects things like the text that appears in text frames.
The reason none of the switches are shown in the Font Styles dialog when you edit the ‘Default text font’ font style are that this is the parent font style, from which others inherit, and because it’s the parent style, you can’t override anything.