dude it did seal the body but i see where u r getting at but the hydra is still a creature and does have a soul and its not like itachi waited or anything but as soon as he saw oro when he said what he was going to sasuke's body he got mad and just sealed him. Preta absorbs chakra and what naruto had was was a natural chakra but the reason i am saying the sword probably cannot be absorbed because its 1. spirtual and 2. it was fuinjutsu with genjutsu powers and i guess nagato did not absorb it during the fight but we cannot be sure if its actually becos he was surprised or if it was becos he could not actually absorb it.I guess, but logically it shouldn't be able to seal dead bodies which lack souls. it cannot seal trees, land, or anything without a soul. which is why itachi waited to seal orochimaru directly instead of just stabbing the head of the hydra. stabbing the hydra wouldn't have done anything since it lacks a soul of it's own. orochimaru has a soul, and he was sealed. nagato had a soul, and he was sealed.
Preta path should be able to absorb the sword. if it was truly a spirit blade, then it couldn't actually cut/pierce through physical matter, but it can. the spirit blade is (probably) converted into a physical chakra based form when equipped to susano'o, making it vulnerable to Preta path; that's why it was physically able to pierce through Orochimaru's mid section. The Death God was truly spiritual, as it didn't actually physically do damage to the person's abdomen during the sealing process. Also, the preta Path has shown to absorb even natural energy (when master-locking naruto).