hmm. i was wondering about that since few years and now u posted a q then i think i should share my idea too, first i thought that this jutsu can be tought just by teaching the hand signs but again u can do hand signs build up chakra but u could not know if u r power full enough to summon the death god EXAMPLE: naruto knew the hand signs for summoning jutsu but could not summon toad because he did not know the amount of chakra needed to summon a particular size of toad so he did it multiple time from tadpole to bunta and gamakichi, same goes for sasuke he learned the fire stayle hand signs from his father but he had to practice a long time to be able to understand the amount of chakra needed for summoning fire from his mouth,so i really do not think that any body no matter how gifted a ninja could learn a teqneque like this just from the scroll or hand sign since this jutsu is always comes as a last resort that means faliour is not an option so my latest guess is that it may be possible to practice this jutsu and summon death god but he dose not seal user,s soul since the caster is not using the jutsu (i mean practicle application like sealing some one) because we can see that boath minato and hiruzen got a sealing mark on there belly after sealing there apponents and after that death god actually yanked their soul so i think it is possible to practice this jutsu without daying well that's my idea and it may be wrong but some how i am getting the feal that the jutsu minato left for naruto to perfect might be this particular jutsu since it is said that a jinchuriki can not cast this jutsu so somehow naruto may be able to cast this jutsu and insted of him death god takes kurama with him and seal it with tobi since he really want the nine taills it will be best if naruto uses that jutsu on him and that sucker got to become a room partner of kurama he he he he.