You can't just say it's impossible dude, Orochimaru was using a forbidden jutsu when fighting the 3rd which brought the 1st and 2nd hokage's body to life but under his control and he was about to bring the 4th aswell but the 3rd stopped it from happening. So please STOP saying that it is impossible for the 4th to be summoned, it will be a forbidden jutsu...
Also, it never showed Jiraiya dieing, go back and read. He was thinking as he sank to the bottow of that lake and left him there. Jiraiya was a sage and one of the 3 senin, who knows what could happen afterwards. After all, people said Orochimaru is dead, but he's still alive but barely, acting as a virus to take over Kabuto's body.