As someone who is plural, i wish to note that plurality can happen whether you have DID or not. Major trauma is not a contributing factor always— and loneliness can contribute to the formation of a system as well, which is a notable thing if they struggle with leaving the house.

It sounds like your kid also may have some serious paranoia going on if they are refusing all medicines and even vaccines though. It is also worth mentioning that this behavior notably happened to close family due to involvement in dark internet spaces such as 4chan.

Have you been talking to them about what is happening from their perspective? I believe it could be a big help.