I greatly appreciate Critter-3's comments, but with all due respect:
1) I used to be a tax lawyer, now long retired. The law has changed since.
2) The blue link takes one to the requirements for "dependent" which what comes before the link clearly states is irrelevant to whether the child (or other person) is a qualifying person.
As it happens I have PhD in nationality law. I am convinced that the child could indeed qualify as a U.S. citizen (the issues are complex and relate to proof of prior presence of the mother 50 years ago in the USA, and the relevant points are dealt with in an article entitled "Citizenship and Unwed Border Moms" in the Foreign Service Journal, and in a 2017 judgment drafted by Ruth Bader Ginsbug in the Morales-Santana case. But for lots of reasons, particularly the child's disability living allowance and disability trust (i.e. special needs trust) in our home country, and PFIC taxes (you don't want to know, unless you do already, about IRS form 8621).
The only case I tried in U.S. Tax Court I won. This matter won't get that far. Either a noncitizen, nonresident grandchild totally supported by a U.S. citizen grandparent is a "qualifying person" for Head of Household, or he/she is not. From what I can tell the return can indeed be e-filed, although in recent years I haven't because I only now found out how to reverse the entry into AGI of Social Security and Canada Pension Plan income (Write the taxable total SS & CPP amount as a negative figure on Sch. 1 line 22b and Form 1040 line 8z). In the past I used pen-and-ink and sent in paper returns.
As a final note, IRS won't receive the working papers and it is more than likely they won't ask for supporting documents. They certainly won't ask me for a legal brief or to explain a TurboTax anomaly (of which there are many). If I were using Intuit's Lacerte (the professional version of TurboTax, overriding the software would be easy.
I invite more comment, especially if somebody can prove me wrong.
