I have reviewed your topic and I believe some clarification is required.

You posted from the IRS: "You were not eligible for a payment if any of the following apply:
• You could be claimed as a dependent on another taxpayer’s 2020 tax return"

But that is why someone would not have gotten the advance Payment. That is based on the 2020 or 2019 tax return on hand, as a projection for 2021. The person you are considering needs to qualify for the Credit based on 2021. Nothing now is based on 2020.

There is the question of If you can be claimed as a dependent. Not Will You be claimed as a dependent. Your person does not seem to meet their own basic needs and seems to qualify as a dependent as a qualifying relative.

That means trying to file for him as independent will show he is not qualified for the EIP 3. The parents would have him on their 2021 return; or, at the least, his own return will have that boxed marked: Yes, someone can claim me as a dependent.

And "and to put him 'on the books' for benefits going forward?" refers to what benefits, going forward? And which "books?" They either want to be part of this or they don't. There is no work record, so no Social Security credits; no survivor benefits. This is sad, but it's their choice.