Re: The Final Villian we all missed. How he's been hiding right under our "noses"
If we assume that the Sage wrote the tablet
Well to this I would say it's still unclear who wrote the tablet or if it was even written by one person. Most people would say it was the Sage and this could be, and probably is true, but because it is riddled with inconsistencies it is hard to tell. It speaks about two halves working together to become whole, then tells an Uchiha how killing their friends will unlock the MS, it's just doesn't seem like thoughts coming from the same person, but we will see.
Does this assume that the Sage was also under Sojobo's influence?
Like I said in theory, I believe that Sojobo is that Sage's father. Now whether the Sage knew who his father was or not I don't know, but let's assume he did. It would seem natural that Sojobo would have some influence over his son whether it be directly (through the Tengu's mastery of illusion) or just because of the nature of the relationship as father and son. King Sojobo was said to have a human pupil named Ushiwaka who he trained in order to see him go out and cause more wars (although the pupil was unaware of this). This could parallel quite well with Sojobo and the Sage, with the Sage being Ushiwaka.
Again, it is still unclear just how much influence Sojobo had over his son, but one thing I think is apparent is that he had no influence on the Sage in the later part of his life, namely, when we see him talking to the Bijuu. Looking at that conversation, it is obvious he lived a life of regret, I assume this to be because he sought "peace through power" as his father and elder son had (remember the elder son is the Uchiha ancestor who's powers are directly related to Sojobo so it would make sense that their ideals are similar). This is also why he chooses his younger son on his death bed, since he had tried the elder's and failed.
The one thing you brought up that I just don't know the answer too is the Bijuu's fear of a fully revived Juubi. The answer I can come up at the moment is the Moon's Eye plan. Like I said in the theory, I believe Project Tsukuyomi to be Sojobo's plan and remember that it is something that requires a full revived Juubi. Perhaps the Sage knew his father's plan and if that is the case, "a fully revived Juubi will signal the end of this world" seems to make a little more sense. The Juubi's power in Sojobo's hands would signal the end of the world. Also, don't forget the Sage told the Bijuu that they would be together again. This is something he seemingly said with confidence and without fear, almost like it was good thing. But hold on, he also said the Juubi's revival would mark the end of the world (as stated through Hachibi)...so which is it?
Well...its both.
The Nine Tailed Beasts were once one being in the form of the Juubi. This as I've theorized is basically the personification of Nature. So when all is said and done the Nine tailed beasts must be set free and put back together to restore true balance (balance is not splitting up the nine tailed beasts between the villages, this is "fake" balance). The Sage knew this, hence, "you will all become one again some day." If it is done right, through Naruto this is true.
Now the other side of this, "the Juubi's revival will signal the end of the world" is also true, if Naruto is not able to make peace with the Juubi and restore balance. This is where Sojobo wins. For if the revival is used for Project Tsukuyomi, it will signal the end of the world.
Was not planning to type that much, but I just got carried away. Hope I was able to cover everything.