Mr. Sohta coming back as a cat rather than a baby like his students makes sense since his crimes were much worse than theirs, so he'd have to live through a life lower down on the sliding scale of reincarnation before he could be reborn as a human again.
Aki refuses to sprinkle salt on Yoshino while she's being beaten by Misao copies because it would be "too horrible." Why? Well, she must be covered in wounds from being beaten so badly. Putting salt on them would only make her pain worse.
Not only that, but salt in many cultures is used to ward off evil spirits, said cultures including eastern religions like Buddhism and Shintoism. It's likely that sprinkling salt on Misao, who is shown to be in the process of becoming an evil spirit herself, would horribly hurt her or even exorcize her. It's telling that Aki chooses to hit her with a bat instead, since she wants to save Misao. Akito does use the salt, but instead spreads it around Yoshino so that it wards Misao away from her instead of hurting Misao.
In all versions but the 2017 remake, you need to find six body parts. Misao's head, arms, legs, brain, eyes, and heart. Where is her torso? Rumor has it Onigawara sacrificed it (and any other missing parts) in advance, but given the way Sohta treated Misao before he killed her...