I think you're over-thinking it, Chris lol. Singular they is singular in that it refers to one person, but you still conjugate(?) verbs as you would as if you were using the plural they.
Hannah went to the store, where they are an employee.
Jack and Jill went to the store, where they are employees.
So even tho Hannah is one person, "where they is an employee" just SOUNDS wrong lol. Which is why you conjugate stuff as if it were plural, even though you aren't referring to more than one person. People use singular they all the time, because we all know "his or her" and "she or he" is stupid and clunky and we naturally use "they" instead, even if using both pronouns like that is "more correct."
If you have a nonbinary person who actually thinks of themselves as multiple people (such as someone who is plural), then they're going to be using "we" or "I" to refer to themselves collectively (because not everyone is cool with multiplicity, so they just go with "I" to not have to justify their existence to others). But this would also be the case of someone who uses binary gender pronouns, too.
Hannah went to the store, where they are an employee.
Jack and Jill went to the store, where they are employees.
So even tho Hannah is one person, "where they is an employee" just SOUNDS wrong lol. Which is why you conjugate stuff as if it were plural, even though you aren't referring to more than one person. People use singular they all the time, because we all know "his or her" and "she or he" is stupid and clunky and we naturally use "they" instead, even if using both pronouns like that is "more correct."
If you have a nonbinary person who actually thinks of themselves as multiple people (such as someone who is plural), then they're going to be using "we" or "I" to refer to themselves collectively (because not everyone is cool with multiplicity, so they just go with "I" to not have to justify their existence to others). But this would also be the case of someone who uses binary gender pronouns, too.