He never revived "dead Kage", he called their spirits (temporarily obviously), which isn't really farfetched seeing he is a powerful spirit himself. And why do you keep mentioning him being a "lingering spirit" as if it was some completely farfetched theory when it is a fact he is one?
There is a reason for the way I write. Bluntly, it's because I have a deeper knowledge of what is going on and am drawing attention to key phrases you should be googling alongside "sage," "Six Paths," "Eight Trigrams," "Tomoe," and other such phrases that would be of interest to you if you wish to understand what Kishimoto is saying.
Unfortunately - it is essentially lost on western audiences because they see it as a bland sitcom devoid of artistic expression and relevance ('cartoons' in Japan have long been a means of expressing symbolism and conveying complex ideas since before the days of the printing press and comic books).
I can't force you to learn. Only show you the way:
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"What is the image of the ideal person, the sage (sheng ren), the real person (zhen ren) in the DDJ? Well, sages wu wei, (chs. 2, 63). In this respect, they are like newborn infants, who move naturally, without planning and reliance on the structures given to them by others (ch. 15). The DDJ tells us that sages empty themselves, becoming void of pretense. Sages concentrate their internal energies (qi). They clean their vision (ch. 10). They manifest plainness and become like uncarved wood (pu) (ch. 19). They live naturally and free from desires given by men (ch. 37) They settle themselves and know how to be content (ch. 46). The DDJ makes use of some very famous analogies to drive home its point. Sages know the value of emptiness as illustrated by how emptiness is used in a bowl, door, window, valley or canyon (ch. 11). They preserve the female (yin), meaning that they know how to be receptive and are not unbalanced favoring assertion and action (yang) (ch. 28). They shoulder yin and embrace yang, blend internal energies (qi) and thereby attain harmony (he) (ch. 42). Those following the dao do not strive, tamper, or seek control (ch. 64). They do not endeavor to help life along (ch. 55), or use their heart-mind (xin) to “solve” or “figure out” life’s apparent knots and entanglements (ch. 55). Indeed, the DDJ cautions that those who would try to do something with the world will fail, they will actually ruin it (ch. 29). Sages do not engage in disputes and arguing, or try to prove their point (chs. 22, 81). They are pliable and supple, not rigid and resistive (chs. 76, 78). They are like water (ch. 8), finding their own place, overcoming the hard and strong by suppleness (ch. 36). Sages act with no expectation of reward (chs. 2, 51). They put themselves last and yet come first (ch. 7). They never make a display of themselves, (chs. 72, 22). They do not brag or boast, (chs. 22, 24) and they do not linger after their work is done (ch. 77). They leave no trace (ch. 27). Because they embody dao in practice, they have longevity (ch. 16). They create peace (ch. 32). Creatures do not harm them (chs. 50, 55). Soldiers do not kill them (ch. 50). Heaven (tian) protects the sage and the sage becomes invincible (ch. 67)."
What do you mean with Naruto knowing where BZ was hiding? Naruto knew that where was this evil servant of Madara, and thatt's what Hagoromo saw through Naruto. He had no idea of BZ being Kaguya's trumpcard for revival.
Come now, let's think about this.
Naruto was able to sense where Black Zetsu was. Using abilities very similar to the Sage (and abilities the Sage could be said to have as the Jinchuriki of the Ten Tails).
How, then, could the Sage have missed Black Zetsu?
The only person to claim Zetsu was unnoticed by the sage is Black Zetsu. He seemed rather adamant about excusing the Sage - so that leads me to suspect that Black Zetsu is -not,- in fact, a creation by Kaguya. He might be created -from- Kaguya - but not -by- Kaguya.
The alternative is that Black Zetsu was not unnoticed but simply allowed to be.
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You will find a more lengthy explanation and supporting evidence there.
Kaguya turning evil from power is strange, but Hagoromo, who never ever was shown to have a hidden agenda at all suddenly being revealed as an evil villain isn't?
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This is the most likely explanation for why Kaguya suddenly changed.
Hagoromo, in his attempt to relieve his mother of her suffering (or perhaps for some other reason), sealed away her compassion for humanity so that she could leave the Earth behind and finally take her place amongst the divine beings.
Her compassion, however, was keeping her from carrying out the duty that was assigned to her. Every thousand years, an Ototsuki consumes the fruit of the God Tree tended by the Uzumaki. The Ototsuki then uses the chakra to gaze upon the Dao/Tao using the Byakugan, before ascending to the realm of divine beings.
With the power of the fruit distributed amongst her children and humans, and her compassion for said creatures removed in the attempt to get her to let go of things that caused her pain - she set forth to reclaim all of that chakra and fulfill her duty (although the whole army part might also be revenge against the Ototsuki for bestowing a duty upon her that caused her such grief).
These two threads also explain in great detail why the Sage has always been GeDo - Misguided.
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There are two others you might find of interest along these same lines:
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Some of the older threads are quite antiquated and I've learned a lot more since then.
Of course - this is the thread 'to rule them all.'
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That is the theory that was compiled out of over a year of research, discussion, and debate.
What is unfortunate is that western audiences have absolutely none of the ground work to really begin to understand the symbolism and the deeper message. They simply see "Kishimoto ripped of DBZ" or "It's just like [insert movie]."
I understand why this is - Americans have been fed sitcoms, movies, and cartoons with self-concluding plot lines where the only expectation is depictions of violence, humor, or a combination thereof. The idea that a comic book is used as a means of communicating philosophical ideas/concepts is about as alien to us as Moon Rabbits are.
Of course - even a cursory look at Japanese culture will reveal the reverence for Princess Kaguya, the Moon Rabbit, beings like the Kitsune, etc. It's all there if you want to learn about it. You just have to be willing to accept the idea that the 'comic' was written for a different culture intending to convey deep meanings. Therefor you have to study the culture enough to catch the references to decipher the meaning.
