This answer contains spoilers… (view spoiler)[I got he impression the narrator thought that neighbor mother had lied to him about the circumstances of Chibi's death. The fact that she supposedly died in the same spot she'd originally been found, that she was killed by a car where not many cars were to be found and if they were, they were going slowly enough that Chibi would have ample time to get away. I thought it was her (the neighbor mom's) way of excluding the narrator and his wife from Chibi since they (narrator and his wife) intruded into their lives with their previously unknown relationship with the cat. (hide spoiler)]
Susanne Haywood I'm really not sure either, but wonder whether he just wanted to express the inevitable doubts and fears that you would have if an animal you loved died without you being there. There would always be more questions than answers.
This answer contains spoilers… (view spoiler)[The fact that they were not at home on that day, and thus created a new and unusual situation for the cat during which it lost its life, is how I understood the last thoughts. "If we have not left our home that day, the cat would still be alive" might be one of the thoughts crossing author's mind. And yes, they will never know the truth. (hide spoiler)]