Uhm, again, he doesn't give you a reward for teaching him the song.
He gives you the Heart Piece as a way of befriending, because you know Saria's Song.
Let me quote the dialogue:
That's all he says in regards to Saria's Song.
And I dunno about you, to me, it means he wants to be friends with you, simply for knowing the song.
Then again, it's rather vague.
So it could mean anything. ^^;
The Mask thing is the only thing they have in common though.
Nowhere in OOT does it say that the Skull Kid has trouble with friends. oO
He only exists as a minor quest for two rewards and really has no story in that game.
I dunno what the Moon and its illusionary world have anything to do with this.
But as for the face-argument, while it does make sense at a glance, we don't really know if that's really the reason Skull Kid wears Majora's Mask.
If anything, I think it was strongly implied, he wears Majora's Mask, because he was slowly falling under its influence, to serve as a host.
Game Theories are like Conspiracy Theories.
Crazy things people pull, that do sound plausible at a glance.
And just because it sounds cool, people immediately believe it.
Game Theories also completely ignore the fact, that games are games, created by people.
And everything that goes with it, like oversights on the devs, lack of vision and other things like that.
Let me bring up that one interview again, where Mr. Miyamoto and Mr. Aonuma were asked, what planet Hyrule is on.
Mr. Miyamoto responded with "Earth".
While Mr. Aonuma said afterwards "I don't know. Let's just roll with it."
Which clearly indicates that Mr. Aonuma never thought about Hyrule beyond the realms you can see in the games.
And thus, all theories on world building are instantly rendered moot, because it's ultimately up to personal interpretation, if the creator didn't bother thinking about it.
There simply is no right or wrong answer there.
Bottom Line: Game Theories are stupid.
And people that actually, genuinely believe them without question, most likely are just as crazy as people that believe crazy conspiracy theories.
Yes, you get the Piece of Heart by showing OoT Skull Kid you are friends of Saria, because you know her song. But the MM Skill Kid also knows the forest song. The fact he knows a special forest song taught to him by a "fairy kid" means he's from Hyrule, whether or not Saria or Link taught him the song. "Fairy Kid" implies a Kokiri (whether genuine, or Link thinking he's a Kokiri).
The skull kid flees from you before he knows you're friendly. So he's already distrustful but lonely. The other skull kids are the ones from the ocarina minigame that gives you another piece of heart, those are friendly (to kids) enough they aren't candidates.
The lone skull kid wanting a new face, which you buy from the Happy Mask Salesman. The HMS is the same one you see in MM and Skull Kid steals from him in the forest that's inbetween the dimensions/still in Hyrule. Since after Epona gets captured do you find the portal to reach Termina. So the Skull Kid is used to slipping through the dimensions too. Feeling lonely from his giant friends probably made him want a 'new face' again. Probably a 'scary face', even scarier than the skull mask he had. Not surprising he became easy prey to the scariest mask on the sale pack.
So they're trying to say it's the same Skull Kid while keeping it vague enough to act as stand-alone so newcomers don't get confused.
If you're speaking of Hyrule Historia, take it with a grain of salt.
Personally, i don't really buy the timeline that is presented there.
It feels impulsively spat out, and not carefully thought through.
Even in the Historia itself, Aonuma confirmed that stories are mere afterthoughts.
Not something they carefully considered and crafted.
They always focus on the Gameplay first and after that, they just wing it with a story that works.
Of course, there are obvious sequels in the series.
Like Zelda 2: Adventure of Link (sequel to the original LoZ), Link's Awakening (sequel to A Link to the Past), Majora's Mask (sequel to Ocarina of Time), etc.
But most games in the series are stand-alone stories, that don't really connect with other games.
Twilight Princess being the best example, a game that works as a stand-alone title, with its own rules, apart from all other games.
But connect it to Ocarina of Time via a timeline and some things suddenly start to no longer make sense.
Even if story comes after gameplay, they were paying attention to story and overarching continuity since the beginning.
- Zelda I - start
- Adventure of Link - immediate sequel
- Link to the Past - Zelda I prequel
- Link's Awakening - sequel adventure to aLttP
- Ocarina of Time - connected to aLttP through the wars in the prologues
- Majora's Mask - OoT immediate sequel
- Oracles - blanking honestly
- Wind Waker - 100+ years later OoT sequel
- Twilight Princess - sequel to the other half of OoT
- Minish Cap > Four Swords > Four Swords Adventures line
They were doing stuff even before the official timeline. Plus, the Hyrule Historia was said to change overtime as they see fit. They already altered it by switching the places of Link's Awakening and the Oracle games and adding in Triforce Heroes as a canon sequel to Link Between Worlds. Also, Four Swords Adventure is down the child timeline, it's after Twilight Princess, eventually. (And BotW and it's sequel will most likely be placed after that because of what they're doing)
Actually, while most are stand-alone stories, it's more like 60-40. There are a lot of sequels, even if several aren't directly sequels, like how Spirit Tracks is a sequel to Phantom Hourglass, but it's 100 years later following Tetra's granddaughter instead of the same Link in his next adventure.
Twilight Princess is also a terrible example. Yes, it's a fine game when taken stand alone, but that's one game that is a distant sequel to Ocarina. The Hero's Shade is the Hero of Time. His regret is no one knowing/believing him saving Hyrule from Ganon in the game proper so he's teaching his Twilight Princess descendant sword skills he picked up over the years afterwards to ease his spirit.
Though if you're unsatisfied with the official one, I recommend the fan timeline that's coming back into some popularity. It essentially fixes the fallen timeline (that's honestly the most "slapped together" part) while also fitting in the Four Swords series in better. NintendoBlackCrisis showcased it recently.