Dealing with conditional sentences can cause a few headaches to both native speakers and learners. As far as I’m concerned, you are right in your assumptions: the time code is C1, and its past form is C1′. This is my assessment as a native French speaker, but I will try to explain myself.

The first thing you might want to avoid is to strictly compare how both languages deal with a situation. In French, the conditional mood is used for conditional statements, whether the time of occurrence is present, future or past. As a mood, it has several tenses (present and two forms of past), as in this Camus-like example:

S’il n’y avait pas maman, je prendrais plaisir à me promener. (present conditional)

S’il n’y avait pas eu maman, j’aurais pris plaisir à me promener. (past conditional)

We could try to find an English equivalent, minus the style:

If it was not for Mom, I would enjoy a walk. (present)

If it had not been for Mom, I would have enjoyed a walk. (past)

The irreal statement here did backshift both in French and English, which contradicts what you are saying in your question. I am no English specialist, but isn’t backshifting only ignored in the "I wish I were…" form? I think it is, because that form is often seen as obeying a special rule.

Let us focus on Camus again, this time with the actual line from the book:

Il y avait longtemps que j’étais allé à la campagne et je sentais quel plaisir j’aurais pris à me promener s’il n’y avait pas eu maman.

I put the second half of the sentence in italic, because it is the same proposition as the one in my past conditional example. Which means that if Meursault were telling his story in the present tense, we would read:

Il y a longtemps que je suis allé à la campagne et je sens quel plaisir je prendrais à me promener s’il n’y avait pas maman.

And we are back to the past conditional example.

In a nutshell, here are the answers to your questions :

1. The time code you assumed is right. I would add that there is indeed a doubt on what Meursault means by "Maman" : the act of her dying (-2), or her condition of being dead (-1). I guess only Camus can answer that.

2. Backshifts happen in French… but I do think they happen in English too 😉

3. Well, no deviation then! Unless for "Il y avait longtemps que j’étais allé…", which looks odd in 2015. We would rather say now "Il y avait longtemps que je n’étais pas…".

I hope I was helpful!