1.) Jiraiya is dead. He chose to die and reveal Pein's identity with the code on Pa toad's back instead of escape, which was offered by Pa toad. Summoning Jiraiya would be a forbidden jutsu requiring a human sacrifice. Sure one can dream he's alive, but it's not a prediction.
2.) It's possible that the flaw in the 50% FRS used by Naruto against Kakuzu was that it damaged the user's arm in addition to the enemy. It is quite possible that upon completion that is no longer the case, not that it simply does more damage and sage mode is required to mitigate that.
3.) Sage mode may be exactly what Naruto needs to increase the number of tails he can withstand. More 9-tails chakra with not lead to too dramatic an increase in chrakra proportion. Also, sage mode will better protect Naruto's body from the Kyuubi chakra. Even at one-tails, Naruto is so much harder to fight. The chakra doesn't move necessarily in the same direction of Naruto himself. This caused Sasuke's full pre-MS Sharingan to only be able to predict Naruto's physical and not Kyuubi chakra movements. Kyuubi mode also protects Naruto as sort of an equivalent to a constant-no blindspot-Hyuuga rotation defense jutsu. This is already known by almost everyone here though, so sorry for the repeat. The point is Kyuubi mode is very powerful, and there are two reasons sage mode makes it possible.
4.) I'll keep saying this, Naruto and Pein and going to knock each other out, and their bodies withdrawn by an ally/s. After that, Naruto will return to Myokubozun (toad mountain) where he'll work with Pa frog and the toad scroll to harness the Kyuubi.
b.) Jiraiya et al. know that the seal is weakening and will break. Pa toad will either work with Naruto to remove/break it, or they will design a new seal allowing Naruto control over how much he draws in/out. Right now his anger/Kyuubi really controls it, a new seal would give Naruto control over how much to draw out. That may be the jutsu the fourth left for Naruto to complete. That's far more powerful than FTG. Also, Naruto won't need to work with the Kyuubi since he'll control him via the seal.
5.) Why Naruto is the destined child. On the last page of 429:
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It says "Pain used his pain to crush Konoha!! As the cycle of hatred continues will Naruto finally... finally return?"
Naruto has the power to break the cycle of hatred, which is why he is the destined child. I'm not claiming he does it now, but he will start now by defeating whatever is left of the bodies Pein is using at Konoha.
