grammar - What is the grammatical subject in these phrases: "what is there to eat?" and "who is at the door"

If I say "there's something to eat, most analyses I have seen seem to imply that the pronoun "there" is the grammatical subject in similar existential sentences (but not all sentences with "there + be", What's the subject of "There is my biscuit!" ? And how about "There is one biscuit left"? is an interesting discussion of this). I also understand that "what"and "who"are the grammatical subject when used to begin questions and that sentences can't have two subjects (compound subjects not being the same thing).

Can someone please explain what I'm missing here?