there was a narrator in the anime, but his role was cut down quite a bit, and matched more of the manga's narrator, except the last episode where they had it closer to the LN's.
To note with your one bit about Mia doing this mostly for herself. She would actually have tired to do things, she isn't as self centered as you might be lead to believe. She actually does care about others, but her main priority is always herself. So if she sees that bettering stuff for someone...
nope, she's very much still a 12 year old mentally, regardless of how much she tries to convince herself otherwise. Basically, all she got by going back to the past is the fact she has the memories of the future, she's still very much mentally the same.
The anime was more heavily based on the manga in the first half, while the second half is more based on the LN, so they had to come up with how things looked and played out. As an adaption of the LN for that stuff, it was fine, but it also resulted in a lot of things being skimmed or missed.
there's a lot with why he was involved, and it's stemmed from how Sunkland's mindset it, which was kind of touched in this chapter but is a bit more explored in the LN.