About this "entering the sage mode" thing: Why would Jiraiya even bother to try entering sage mode on the battlefield in the first place? If it really depended on it, he can always kuchiyose Bunta and say: "go tell Fukasaku to reverse-summon me to Mount Myoboku - I need to enter sage mode there", Bunta goes *puff* and Jiraiya can now expect being summoned to the toad village any moment. It might be quite and interesting strategy too.
Imagine Itachi launching his Amaterasu and just a milisecond before it hits Jiraiya goes *puff*

Itachi now cant do anything but wait for him to return in sage mode and with frog army. He can even leave a kage bunshin fighting and avoiding getting hit as long as possible so Itachi wouldnt notice. Then, after he entered the sage mode, he can be summoned back to the battlefield along with pa and ma, bunta, hiro and ken, voilá! We MUST NOT forget that Mount Myoboku is at his service all the time (Itachi has his artifact tools, Jiraiya has this "partnership" with toads

)
I hope this clears the problem of entering the sage mode.