Where does it say that he didn't come up with the understanding of ninjutsu and chakra till after he became Jinchuriki? Cause I honestly don't believe that.
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Look at this page. It specifies that he appeared to teach everyone the divine power of chakra and he didn't get freat power until after he became the Jinchuriki. I am not going to find all the pages for everything.
Hmm about this I don't think it is a valid argument, since it was the SO6P who created the 9 tailed beasts. So he must have given them the powers as well. The 10 tails is not comparable to these 9, since it consists of all of them. Perhaps SO6P had all these abilities as well, but other shinobi has been able to use some of the same abilities. I think the SO6P gave them these abilities, since it would make no sense if they belonged to the Juubi (ink for instance wouldn't make sense).
It is said in the Manga that the powers of the Bijuus are not just the power of the Juubi but also that all of the powers were of the Juubi as well. The Juubi was comprised of Pure Natural Energy which can be made into basically everything with the power of Creation of All Things. Who knows really me or you can be right because it has been said int he manga that they are not just split chakra but also the powers are split from the Juubi as well. You can also be right because of the forms that Rikudo Sennin made them to be. So why would he only give Kurama sensing of Negative energy and healing?
Again there is no evidence that sharingan came from the Juubi. The Juubis eye doesn't look that much alike it. The Juubi has 10 spots in the eye, which is more likely related to its number of tails. The circles in the eye does, however, resemble rinnegnan, I will give you that, but it is being controlled by rinnengan users in all these illustrations, though it's impossible to say, whether this is related or not. Nonetheless there isn't much evidence for this matter, in fact I find more against it than for it. We haven't seen any official coloring of the Juubi yet, but most draw it with a red eye, which of course is related to sharingan then, but still need some actual valid proof for it.
I am sorry but you cannot deny that when the Juubi himself has both the Rinnegan and Sharingan eyes mixed. Plus you need both the Sharingan and Senju DNA just to get the Rinnegan which the Juubi obviously have. Unless you are telling me that the Juubi is the summon of Rikudo Sennin. Which in my eyes would not make him the saviour of the world but someone who was trying to clean up his mess. Also if he had such great power before the Juubi woulnd't he have had the power to stop the War way before he defeated the Juubi without needing the Juubi's power??
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Here is the Manga where you can obviously see the eyes used to use Creation of All Things which just so happen to be the same eyes of the Juubi. Is that coincidence?? I think not.
I am pretty sure it was said somewhere in the manga that he invented some sort of sealing technique to seal the Juubi inside himself. If he did this before learning all those things, it would make no sense, unless it will be explained in many details. Perhaps the Juubi accepted him, and wanted him to bring peace, but as for now it seems unrealistic that we will ever know. It is pretty evident that he must have mastered many of these tings before becoming jinchuriki of the Juubi, but I cannot disprove that he would need rinnengan to do it. Sage mode, grand ninjutsu and sealing techniques, all these could probably do it, but there is just no way that he got these from the juubi itself. Hence unless the juubi wanted to be sealed.
But why couldn't the sage himself be the product of 2 former war clans that had been the main clans in the past world. This product itself made a person with a unique chakra and unique eyes. We cannot look away from this possibility since a child with his origins from uchiha dad and senju mom hasn't been seen yet.
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Here is that page that you were looking for and a powerful technique to seal anything is called Fuinjutsu. We can get an idea of the technique used on the Juubi from the same sealing technique that Naruto used on Kurama. Kurama himself thought that it was Rikudo Sennin.
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It says right here that they were not led down the right path when they were in him so the Juubi did not just agree to go inside him to create peace especially when he was a blob of hatred and negative energy.
How would he have Ninjutsu to defeat the Juubi if he created Ninshu which became Ninjutsu after he defeated the Juubi. He defeated the Juubi to protect the world then he went around the world spreading his religion Ninshu, to stop the wars, which became Ninjutsu. I believe that he was a Master Fuinjutsu and Sage Mode, all which used in the most efffective ways can defeat a lot of anything especially when the Juubi is made of Pure Natural Energy. Maybe he manipulated the Juubi's energy somehow by being a Master of Natural Energy somehow. The powers of Nature energy go much farther than just enhancing your physical abilities but also give advanced users the ability to see in the future like how the Great Toad Sage does.
As far as your question as to why no one including the Sage couldn't be a product alone be the product of both the Uchiha and Senju is that all the attributes of both Clans are definitely inherent of the Juubi eyes and all. You can't believe that Rikudo Sennin who housed basically all the energy of the world inside of himself couldn't gain power of Great Eyes and Great Body to pass it on to his sons without still being great because it isn't all of his power??
Come to think about it I think that there is some kind of connection to the Senju and Senjutsu Mode that Naruto uses. Why? Because just like all of Naruto's abilities are just enhanced the same happens for a Senju. Maybe the Body that everyone is talking about is a product of the Nature Energy that the Juubi was made of. So it would stand that the younger brother got very powerful Nature Energy infused within himself naturally and the elder brother got the eyes and chakra. Senjutsu energy would definitely make a clan like the Senju naturally stronger than any other clans just like how Senjutsu Energy enhances all of Naruto's Abilities. That would also explain why Hashirama has the ability to use a powerful version of Moukouton(Just like how Kabuto used his ability from Jugo's Clan to move the earth) while also able to heal without hand signs(Just how Kurama heals Naruto by being only Yang Energy-Physical Energy-Body). WOW!!!! It would also explain why when anyone gets Hashirama's cells that their power is automatically boosted 10 fold just like how Sage Mode does to Naruto. IDK I think that it makes sense and is very much connected which would also explain why it is so much easier for Hashirama and anyone with his cells are able to control the chakra of a Bijuu so easily.