I'd just like to remind you that the first time Naruto did the rasengan was not while training to do it but against kabuto. It was in a real battle that he mastered it. The kid banked on teh fact that he could do it without doing it before. naruto's confidence in himself is the only reason he is the protagonist. This confidence makes him never give up. What I am trying to say here is that naruto will be able to do the tailed beast bomb in actual battle now (i guarantee it). The clue to this is when bee says you 'almost' had it. The first time he does it will be in battle.
The reason Naruto is going to battle is because he doesn't want people to die protecting him. The whole reason why he has all this power is to prevent that. If he is the only man left alive when he is allowed to fight madara, there is no point in living. he is protecting people important to him. also remember how the 4th sealed the kyuubi in him in the first place because he realized no normal ninja could take out madara. Now that the Kyuubi has been harnessed, naruto is the only one who can save the others. Otherwise no matter how they try, eventually everyone will die and even if they don't then the cycle of hatred will keep going.