Some things to add:

From Pantheon's bio, we see that the Rakkor have been known to fight with Demacia and Noxus on a regular basis, and both of these city-states have been around for hundreds of years. Noxus at the very least has been extremely aggressive toward pretty much everyone around them, so it's easy to think that they may have been fighting the Rakkor for well over 100 years. You'd think they would notice certain warriors that can call down the power of the sun or moon. Rakkor may be reclusive, but they're certainly not "unknown" to the other city-states, especially not Demacia and Noxus since they kept uselessly throwing battalions at them.

Heck, Leona's ascendance was seen throughout Demacia, at the Institute, and everywhere remotely near Mount Targon. If this sort of thing happened within the past 100, 200, or 300 years, people outside the Rakkor would write about it. This is a completely unique phenomenon we're talking about here, a complete mystery. The sun was shining at midnight. Here's the Journal of Justice Issue where it's talked about. They didn't say "Oh something like this happened over 100 years ago, we have records and theories about it," they have to go to Pantheon and ask him what's up. And Pantheon doesn't know either. He starts to bring up one possibility, but stops before he can give them anything of value. Probably because even Pantheon doubts that there could be another Sun avatar around. The Rakkor are supposed to consider the Sun avatars to be legend by now, according to Leona's lore. By lore we know that all Solari are Rakkor and from what Pantheon says in the article that they're a very important part of Rakkor society. The Lunari, Solari, and Rakkor are not separate tribes, just various groups in one tribe of people.

Weird sun phenomenon (and moon phenomenon since we're assuming that both the avatars were around a bit over 100 years ago now) should be something that astronomers and mages, if not pretty much everyone who can keep a journal, will write about when they notice them. Mages/summoners have been around for thousands of years. If they saw a strange event that they suspected to be magic----and let's face it, the sun shining at midnight or any time during the night is definitely magical in nature----they would write about it in detail and even theorize about it. Especially if they're constantly trying to outgun the magic on the other city-state's side.

As to the ages of Demacia and Noxus... There have been four generations of the Jarvans, and General Boram Darkwill used necromantic magic to outlive the first three only to be assassinated during the fourth. The two city-states existed before Boram and before the Jarvan line. Before Boram, Noxus was ruled by the Black Rose, and before the Jarvan line was established, the government of Demacia wasn't entirely stable. They would keep records. The League would have access to these records, yet they were completely shocked by sunlight shining on Mount Targon for ten minutes and had no explanation for it.

Now you might make the point that the Sun avatar is only a religious figure, they should have nothing to do with actual fighting, right? What's the point of the functional armor, then? The avatar doesn't get those abilities for nothing, they're meant to be used, and not just for burning heretics.

Ahh, rediscovering the JoJs and old League lore. I can't believe I overlooked the huge spectacle of Leona's ascension and the fact that Demacia and Noxus used to routinely lose battalions to the Rakkor. My mistake.

I believe that Nami definitely shouldn't be a side story in the conflict versus the Lunari and the Solari, but I think that having a connection to Diana and the need for the Moonstone gives Diana and the Lunari credence. It should just be a strong point of added argument to Diana against the Solari who refuse to acknowledge her. It shows that there's a group of people, the Marai, that have historically had a strong relationship with the Lunari and have a role to play in Runeterra.

I bet GP has something to do with things as well. Guy likes to plunder.

It feels awkward bringing up an old topic..

But after reading all of these theories and conflicts about why the Rakkor would want anything to do with these Ocean dwellers who supposedly lived a large distance away...

I have a few things to note:

According to Fizz's lore, this water-dwelling civilization was located underneath a mountain (Mount Targon?). But it was hidden, which may account for its absence on Valaron's map. His lore also states that his civilization had its enemies but had an impenetrable defense (referring to Nami's lore and the moon stone?). However, it also states that upon returning from one of his adventures one day, Fizz finds his people have disappeared and his home has come to ruins. From this, we can assume that while Nami is out looking for a replacement battery (the moonstone) their defence system has already diminished and whatever enemies they had, took advantage of that.

Also, one thing I wanted to note about Diana was that her lore doesn't really mention her parents. I was thinking that maybe the Lunari haven't been wiped out entirely. That perhaps there are still descendants of the Lunari living among the Rakkor and even among the Solari people. So, those of you who say it's impossible for a race/religion of people to be eliminated within 100 years, this is my theory. I mean, with the Solari being like "Eliminate all Lunari!" it's obvious that there were those who rebelled (were probably slaughtered) and those who hid, converted, and blended in with the Solari. Among those people might have been Diana's parents, whether it be her father, mother, both, or even one of her grandparents. Diana is proof that the Lunari still exist.

Sorry for reviving a dead topic, lol, I just got around to reading Nami's lore and ended up developing my own opinion.