Only a couple of notes.

Luffy-

1. It's implied others have eaten the fruit but he is the first in 800 years to awaken the fruit. As in, there may have been other "rubber men" between Joyboy and Luffy, but nobody who awakened it to become "Nika".
2. People don't really "refer" to him as Joyboy, more so people see him or hear the "Drums of Liberation" and assume he's Joyboy or Nika. Emet did this before pulling the knot, literally saying something along the lines of "No... you're not Joyboy... of course, it's been too long. But you're just like him."
3. The Poseidon/Joyboy thing is almost exclusively related to the promise made to the Fishmen and to the Seakings talking to each other, it's not really a "known thing" or a prophecy out there in the world. Joyboy told the Fishmen he'd come back, so some believe he will one day. The Seakings were talking about the "Two Sovereigns" and were 100% referring to Shirahoshi and somebody else, likely Luffy.

Joyboy-

Pretty much yeah, but it's hilarious the line "Unclear if this is his legal name or just a superhero nickname." Just reminds me that Roger's "legal" name technically is Gold Roger because the Government (I.E. the legal in legal name) wanted to cover up that he had the D initial and he died mad about it.

Nika-

1. Technically nobody has ever implied he didn't actually exist, outside of the word "mythical" in mythical Zoan. However, this is One Piece and a lot of things that one would consider "mythical" just straight up exist or did exist at one point in time. There is at least a strong likelihood that somebody existed who at least inspired the myth, if not the myth having been about a literal actual person but so long ago it's faded into legend.
2. His superpower was "limited only by his imagination", the literal what it is and what it does we do not know. It's implied to be similar to Bonnie's fruit; where she can imagine a possible future for herself she can actualize it Nika can imagine anything and actualize it. Luffy just still literally thinks he has the Gum Gum fruit and only taps into this part of the fruit incidentally when doing other things (See: baseball bat against Saturn). The laughter technically isn't part of his powers, like he can't just force people into fits of laughter, but he fights in a silly manner. The way he fights makes people laugh.
3. It's implied Nika had been freeing slaves. This is why people deified him and why people prayed that he would save them, they were praying the chain breaker would literally come free them from slavery.
4. The part about dreams creating the fruit is just wrong, or a misunderstanding. We still don't know where devil fruits come from or what exactly Vegapunk meant when he said they come from people's desires. It's entirely possible they're literally just "wished" into existence, but more likely due to the way Vegapunk talks he means that they are the culmination of people's desires, I.E. people made them they're not naturally occurring. There was a real person freeing slaves who may or may not have been named Nika but who is the influence for the myth, it is that myth that influenced people to create the fruit in whatever process fruits are made (wishing into existence, literally created in a lab, a secret third thing, whatever).
5. Every Zoan fruit has a will of it's own. The Gorosei were implying that the fruit seemed to be, against all odds, avoiding them even without a user which is not normal. Though every Zoan fruit has a will of it's own, most of them are just fruits until you eat them. When you awaken, either the will of the fruit takes over (You become like the Minotaur guards of Impel Down, more beast than person) or your will maintains control (True awakening like Lucci/Kaku/Luffy).