Q:

(L'Étranger de Camus) The usage of "se" with "laisser + faire"

This question is on "se" in the last sentence of the following quote from L'Étranger by Camus.

Nous nous sommes mis en marche. C’est à ce moment que je me suis aperçu que Pérez claudiquait légèrement. La voiture, peu à peu, prenait de la vitesse et le vieillard perdait du terrain. L’un des hommes qui entouraient la voiture s’était laissé dépasser aussi et marchait maintenant à mon niveau.

From English translations, I understood that one of the men had fallen behind and got level with the narrator.
Questions
(1) Should I understand that "se" refers to the man who fell behind.  That is, should I understand the sentence to mean that he "had allowed the passing of himself"?
(2) Should I understand that the implicit subject (agent) of "dépasser" is the car (la voiture) and anything else in the procession between the car and the narrator (moi)?
(3) If we replace "se" with "le," would that be ungrammatical?
My motivation for question (3) comes from considering English sentences like, "He let himself be passed" but "He let her pass him."  I think my (English-speaking) mind wants to assimilate "se dépasser" in the quote to "her passing him" because "se" occupies a position like "him" (in being the object of "dépasser") even though "se dépasser" does not have an explicit agent (like "her" in "her passing him").

A:

You are perfectly right about (1) and (2).
To write a similar sentence in active voice you'd use:

Il les avait laissé le dépasser. (He let them pass him.)

“Les” stands for “them” (alternatively you could use “la”  for “la voiture”). Le stands for “him” but since “he” and “him” are now respectively the subject of “laisser” and the object of “dépasser” there is no need for a reflexive pronoun. French and English constructions are very similar in this respect. 
Note: if you omit the le in the active statement, as your replacement suggested, the reader would be lost as it strongly suggests that the passing/overtaking applies to a third party. In French, the le that stands for the previous subject is generally made explicit. It could be omitted, but only if the situation is very clear to the reader already and the act of passing/overtaking itself is significantly more important than to whom it applies. Also, in any case, if you don't use a reflexive pronoun the auxiliary should be avoir.

