I wasn't really thinking about Tojiko possessing people to complete her immortality process, it was more a "general capabilities of the character" question. I mean, if she still COULD possess people and youkai the way Kanako says vengeful spirits can, she'd be an enormous danger to youkai. Incidentally it seems that possessing youkai would kind of complete her immortality process, since, if Byakuren is right, that'd completely destroy the original mind of the youkai and replace it with hers (Byakuren: "It means they have died as that particular youkai. What's left is a completely different youkai. [...] The new youkai's core would be the mind of the spirit that possessed them... In other words, rather than being reborn, the spirit will take over their entire existence.")
I'd trust Kanako on this matter; there's no reason for her to just make this stuff up (unlike Yuyuko who is fond of trolling), Akyuu, Miko and Byakuren didn't disagree with her (one is a scholar and the other two are highly knowledgeable on spiritual matters, and Byakuren is the one that explains what would happen if a youkai got possessed), and although Kanako does say she never actually heard about youkai being possessed, the way she speaks about humans being possessed and the consequences of it imply she has seen or at least heard about this happening before (Kanako "The scary part is when they possess humans. They possess to humans and induce them to resent their own kind. [...] they would fall into paranoia, not being able to tell friend from enemy.")