1: Well, we don't have a conclusive answer, but you could speculate wildly.
First theory: Check Ch. 11, page 7 + 9. Tooru was abused by his mother, and he set her face on fire. Who knows, maybe she held a grudge.
Second theory: Everything is fabricated since you know, Toorus memory seems to be somewhat missing/altered. Who knows if his mother got him that job, or if some underlying instinct, influenced by his repressed memories, made him apply for the job.
Third theory: A huuuuuge fucking coincidence. (Very unlikely)
2: She noticed Rinko stopping from hearing someone (Sakurako, maybe...) calling him a "Filthy murderer". She simply asked why he stopped ("What's wrong?").
2.1: I figured you might have written the wrong page, and the one you were asking about was page 52 "Filthy murderer". It is implied Sakurako survived (notice the little "wings" in her hair?) and simply calls them out on their past. If this is actually Sakurako, or Rinkos delusions... Who knows? God knows Rinko should be enough fucked up to have some hallucinations.
3: Everything points towards her being blind. Why would she fake it? I guess you might derive this question from page 53 where she looks up towards Rinko? I don't think you should try to read into it any deeper than necessary. While it's possible I guess, I do not see how that would change anything.
4: Sakurako is 14 years old. Rinko is 10 years old (Check character page on Vol 1, Ch 3, page 29), and Shiiru seems to be the same age as Rinko. So no, there's no possibility she's Sakurakos child.