Grammatical role of « en » in pronominal verbal locutions that begin with « s'en »

Question

For example - « s'en aller, s'en ficher, s’en tirer, s’en prendre à, s’en branler, s'en falloir, s'en sortir, s’en battre l’œil, etc. »

Are these to be treated like any other pronominal verb? Is the « en » here playing a grammatical role as a preposition or as a pronoun? Would a construction like « j'en m'en verbe » be valid to replace « je m'en verbe [de COI] » or other such complements (e.g. partitive/indefinite) that « en » can replace?

I'm having a lot of trouble understanding the motivation behind the construction of these expressions and developing an intuition for their meaning. Obviously they are non-compositional, but is there anyway to get some kind of intuition about them at all without memorizing every one?

Answer 1

In all the examples you listed, "en" is the pronoun serving as the COI.

Example: Sa voiture? Elle s'en fiche. (her car? She doesn't care about it.) "en" = "sa voiture" (her car) is the COI.

You could have said: Elle se fiche de sa voiture.

So because "en" is the COI, it's incorrect to say "je m'en (verbe) de COI". Still it's used all the time, for example: Elle s'en fiche de sa voiture. In that case "en" still means "sa voiture". You can think of it as adding a precision after you've stated the main sentence with the pronoun: Elle s'en fiche. (de sa voiture)

J'en m'en (verbe) is never correct, and never used.

Answer 2

In your examples, we say: Je m'en fiche, je m'en vais,... The word "en" is a pronoun for the complement of the phrase.