I think you’ve answered your own question. You write:
“The payee is probably filing a tax return, anyway”
Exactly. So they might not be be filing it anyway!
And you write:
“Burdening the payer of the UBI with the ascertaining what level applies to what person (or who should pay the taxes on an adult dependent’s UBI, etc.,) strikes me as administratively worse, not better.”
But I’m not sure what you’re referring to there, especially as I’m saying there shouldn’t be any tax to pay on anyone’s UBI.