Re. early memories
I know I have at least one before I was two because my parents divorced when I was 2 and I have a very specific memory with them still together. Although, I don't know the exact age. I've noticed that DD seems to remember people and will be much more comfortable around people that she met before even if it was at a very young age (like 5 months old or younger).
OP. DH and I both believe in some sort of afterlife so with DD we'd simply explain our beliefs and answer her questions as best as we can. Maybe you could also explain that it is rare for humans to die when they are young and explain the difference in life spans (human vs. fish). I'm not sure if that would help or not?