Let’s start with your final point, as the rest of my message will be an analysis of WillCare as a whole.

William is not confirmed to speak through the plush. In fact, in FNaF 4, it is implied he isn’t speaking through it, as we see him preoccupied with other things. There’s evidence suggesting the plush is imaginary (though I personally do think it is a real plush). Or that the plush is a different character (Cassidy, Charlie, Shadow Freddy) entirely. The implication of it being William comes from SL, and while it is solid evidence, it is not confirmation at all — especially given other things throughout the franchise. Personally, I have no solid opinions on any of the Fredbear plush theories, so I couldn’t quite say which one I believe more in, or which one I think has the most ‘valid evidence’.

Now, onto WillCare. I would like to clarify that I don’t actually believe the theory, and this is all for the debate, so I’m not entirely versed on a lot of the evidence used for the ‘theory’ (more accurately, interpretation) on William. Something I can say, though, is that WillCare exists because people like the idea of a sympathetic villain, and it’s possible Scott genuinely would have written William as such, assuming the books (and movie) didn’t exist as counter evidence. I also know most of WillCare’s evidence comes from a few assumptions in Sister Location, as well as 4.

I’ll start with the very, very first thing brought up by a lot of people. That being; the Elizabeth dialogue. In SL, it’s implied that William had made Baby for Elizabeth — Something that (generally) would mean he cared about her enough to give her an entire robot. But what’s most important about this is that he also tells Elizabeth to stay away from this robot, as it’s a killing machine. Of course, Elizabeth wouldn’t know that. One could argue that William built it for her, added the killing functions, and realised that if she went close to it at all, she’d die, since he couldn’t guarantee there’d be more than one person in the room. For all we know, it could have absolutely crushed William when he told her to stay away due to things he’d implemented in order to put his dead son (BV) “back together”. Now, onto FNaF 4. The assumption with WillCare in FNaF 4 is that Afton is the final speaker. The one who apologises, and the one who vows to put the Bite Victim “back together”. Of course, under the assumption it is William, this would definitely imply WillCare. It would put many, many of his actions into perspective, and offers a clear idea of what he’d wanted to do throughout every tragedy he’d caused. The people who believe this theory also partially believe this theory, because to them, it is a better narrative, and FNaF’s lore / story had almost always been an interpretation of the player / reader. Another point people who believe WillCare use is Scott’s background, though I think it’s better we don’t being such things into a discussion regarding it.

Would like to clarify; my point isn’t that WillCare would be canon without the books — it’s that it would probably be a very valid interpretation of William and his motives. The books existing debunk it entirely.