And I'm betting most villages DO seal a bodies techniques and buries them.
I'm sure some villages burn their dead, but the manga hasn't shown any examples of it, so it's safe to say it's rare. As for the issues with decomposition, skeletal remains last decades, and DNA can be extracted from bone marrow.
The point about Shikamaru's family plot is valid, but again Kabuto wanted WAY more corpses than what he got. Clearly he couldn't find everyone he wanted.
And again, look at what he got: the jinchuuriki's are obvious, the 3rd and 4th Kazekages were killed by Akatsuki/Orochimaru, Hanzo is obvious, Kimimaro is obvious, Haku and Zabuza...easy, Chiyo-baa had no blood limit powers and was likely buried in the village plot, former Akatsuki members are obvious, etc. Hell, Orochimaru got a hold of the 1st and 2nd Hokages, so both of them were likely buried in the village memorial plot.
None of Kabuto's collection is that far fetched within the context of this story. Madara himself wondered just how much Orochimaru knew/discovered about his own goals in earlier chapters, and Kabuto knew everything Orochimaru did. That alone explains the resurrected former seven swords of the Mist: Madara controlled the Mist village via the 4th Mizukage during their service, and Orochimaru was probably in Akatsuki at the time.