despite not knowing english and nobody speaks it, the pikmin know how to say their own name...

this theory makes me imagine pikmin having a Bugsnax-Style plot twist

I think the Sage Leaf may have just overheard the comms between Rookie and the Rescue Corps. He calls the onions “Gems” before switching to the name Rookie is familiar with.

Edit: The Red Pikmin near Alph can’t reach him when he crashes because he’s in the water.

this theory genuinely makes the story of the game infinitely more interesting, i wish there was a bit more ingame acknowledgement to the seemingly parasitic nature of the onion

Alph wasn't turned into a leafling because he landed in water, one of the pikmin was waiting for him to wake up to lead him to the onion.

I always found found it weird that the moment you use the knapsack item to sleep in pikmin 2 they instantly try to stuff you into the onion

If you speak to olimar before you start the sage leaf trials he tells you about a memory involving a white leafling, suggesting he has met the sage leaf at some point, most likely as a leafling.

I think its interesting to note that Bulbmin have a different but similar method for securing a leader. They are parasites that essentially leafify their main predator, the bulborb, themselves and control all the bulborbs body functions too. I like to think that the dev team studied hard on how insects and other small animals survive to inspire the world of Pikmin

Alph also landed in water, so the red pikmin wouldn't have been able to get him anyway. (Also the fact that Alph is awake to watch the Drake fly overhead and crash after him implies to me at least that he wasn't unconscious for very long, if at all)

"The onions make the ships crash so that they can brainwash the castaways and use them as leaders to the Pikmin"

The onion seeing 11 year old me lose 80 Pikmin to a bomb rock: 🤦🏻‍♂️🤦🏻‍♂️🤦🏻‍♂️

one thing i would like to add is that Louie is seemingly reluctant of leaving PNF-404 in both pikmin 2 and 4, with him saying something along the lines of "i kinda want to stay in here" in 4, and in 2 just straight up not boarding the ship despite being there, (although his reaction in the "credits" say it was closer to an accident), but whether Louie wants to stay due to the Onions messing with him or just him liking being the "king of bugs" is unknown to me

Hi! Quick note on the water wraith thing. The yokai Umibouzu (海坊主) uses the same word as the water wraith (坊主) bouzu, you touched on the multiple meanings of this word in the video (which I appreciate as most people miss it) bouzu written with those two kanji characters always means priest/monk. The other kanji in umibouzu (海) means sea. So while I do agree that the water wraith probably isn't connected to the plasm wraith or part of the reason the ships crashed, I don't think it's realistic to claim that there was absolutely no relationship between umibouzu and the water wraith. Considering the structure of the two names are incredibly similar:
Umibouzu (sea-bouzu)
Amebouzu (rain-bouzu)
Yes, ame has the double meaning but since the entire game is in Katakana we can't rule out either meaning. Honestly it's probably intentionally done that way. Also the fact that it's water-wraith and not candy-wraith in the English translation means that the association with rain/water is undeniably there.

Additionally, Nintendo has taken inspiration from yokai quite a lot when creating monsters/creatures (Pokémon like drowzee being a shining example) so it's not far fetched to imagine they took inspiration from another one when designing/naming a creature.

TLDR: I agree with you that the umibouzu theory isn't likely anymore, but there was definitely some influence with umibouzu in the water wraiths design/name.
(Great video btw, this is just me being nitpicky)

The limit of pikmins you can carry might be the limit of how many pikmins the Onion is capable of connection to the "hivemind"
For example, it's curious that Onions are capable of flight, perhaps the ability to flight could be linked to the knowledge pikmin have over the map? Maybe the onion takes "record" of the area it will land on, and then it connects the pikmin to inform them of routes and stuff

This might be why to imprive the limit we need to feed farlics to the onion, it needs to be upgraded to be able to hold control over more pikmins

In regards to the Sage, he's probably got a universal translator device or something and the translations have to pull from a database of existing terms. Many of the castaways are from different planets and I doubt they all speak a common language.

There is one major thing that needs to be addressed: Why do glow pikmin counteract the Leafifying process? If Onions are seeking out permanent captains, why do the glow pikmin seemingly go against this process? We know that glow pikmin are not officially pikmin, merely having pikmin qualities... but why is this something they can do? Glow pikmin essentially provide food for the captain but also a natural resistance to infection; an emergency escape and life sustainer. Glow pikmin almost lure captains with an opposite method; rather than trap the individual via brainwashing it instead encourages them to stay by providing them with edible matter in exchange for their services. Its perfect for Louie all things considered.

There is a possibility that leafifying is just a last resort for when onions have no other means to keep their captains alive.

this theory makes shepherds pikmin nightmare alot more understandable

here is another thing to look at in pikmin 4 when you 100% the sage cave you have the option to bring the sage with you but he refuses something none of the other leaflings do so the sage could have been on the planet for so long that they are at peace with the planet something that nobody else feels besides louie

the  "mechanical harp (memory song)" has an entry from olimar that states that he feels like it's "sending me a warning about unexpected troubles in the future...", likely meaning that at pikmin 4 takes place at least before pikmin 3.

The good thing is that, even if we end up rescuing almost all castaways, the onions ALREADY WON long ago.

Moss was BORN a leafling dog, which means leafling dogs must be already a part of the planets ecosystem (or at least just dogs as siren hound may suggest) , and seing how she has a natural disposition to lead Pikmin, and remained the 'Captain" of our squad by the end of Pikmin 4, means they are already predisposed to become Pikmin leaders on their own

We can even speculate that, space dogs have helped the Pikmin species to tribe already , explaining to outstanding amount of onions and the wild onionless Pikmin we can find in the game, maybe Pikmin are now facing overpopulation, being why we can't start with 100 max Pikmin on the field right away

my theory has been that Onions are pikmin that have been left in the ground for an extremely long time, so they gather nutrients from the ground and become plump enough to store their kin inside and create more pikmin. But without sunlight they become candypop buds, due to not having as much sunlight, they grow their petals to soak up as much as possible but dont get much.
Similarly I like the theory that a farlic is  an onion that's gotten aged after producing too much pikmin, ready to be recycled into nutrients.
as for your theory I think this one goes hand in hand with that one, though isnt exactly evidence for or against it. Just makes sense even given these theories. Though in general it would make sense, why would a pikmin stay in the ground for a very long time? because nobody is around to pluck the seed out of the ground.