I agree with you for the most part, but I want to address one point.
If Naru was a Sailor Senshi she would have been discovered by Luna a looong time ago as being attacked would have triggered her planet symbol like it did Ami and Rei. Also BEHOLD USAGI'S BEST FRIEND IS ALSO A SAILOR SENSHI! Is one of the laziest plot threads fanfic writers come up with. It's up there with Sailor Earth (which usually goes hand and hand with it )and Usagi leaves Mamoru for Seiya Why can't Usagi's best normal friend just be.....normal?
While I think it's good for Naru to be normal, there are two reasons why fans gravitate toward making her special.
- She's positioned centrally in not only the Usagi-solo episodes, but in the entire first half of Classic even after Ami and Rei come along. The anime invests a lot more time in her character to the point where it comes across as odd when she's dropped.
- In this series, there isn't much that a normal character can do. It isn't as if Sailor Guardians have a special weakness that normal people lack and can save the day to be their mundane selves.
There is also a big hint in the Nephrite arc that she's something more when Nephrite's crystal reacts to her love. Yes, her love is that strong, but why
her love? There is groundwork laid for Naru to do something beyond be a victim...which is conveniently ignored.
I know she loses her narrative function when Usagi's friend collection grows, but it also raises the issue of why did Usagi break away from her normal friends to hang out with her new superpowered friends. As flawed as making Naru a Guardian is, it is at least an attempt to counter the idea that only the special people should hang out with the special people. That Sailor Moon is best friends with her past-life guardians actually works against the whole point of Sailor Moon overcoming her past tragedies and past life to live a present one and make a
better future one.
Personally, I'm surprised more fans don't go the "Ron the Death Eater" route and make Naru into an outright antagonist. I believe a natural development for Naru would be to not only discover that Usagi is Sailor Moon, but to be hurt and betrayed to the point where a villain could corrupt her and override the common sense aspect of said revelation ("Usagi being Sailor Moon means she saved your life!") to have her focus on the negative ("Usagi has no time for people like you.") and force her into a fight. Then Usagi becoming Sailor Moon would mean she would ultimately face the prospect of destroying who she saved to take on the mantle. Even if it were happily resolved, that the question would be raised would be slightly different from "Do past tragedies have to repeat themselves?" or "Does being a Sailor Guardian mean I never stop fighting?" but "WHO am I fighting for?"