Is the infinitive (używać) used instead of either of the third person conjugations (będzie czytała; będzie czytał) because "Kto" is used and therefore we could not possibly know the gender of the person who will be using the machine? I'm asking because in the past with verb lessons, I have seen sentences like "Kto to zrobił? (Who did this?)," in which the masc. third person conjugation is used despite not knowing at all who the person is, let alone their gender. Pardon if I'm overthinking this a bit, but could "Kto będzie używał tej maszyny?" also be a valid way to ask this question as well? I'm just curious for future reference.
In Future Compound there are generally three possibilities for every grammatical person (two gendered and the infinitive), and in every situation, two of them (the infinitive and the appropriate gendered one) will be correct. "On będzie robić" and "On będzie robił" are equally good, same for "Ona będzie robić" and "Ona będzie robiła". One of them shows the gender, the other doesn't, but none of them is worse than the other one.
"kto" is grammatically masculine singular, even if in the particular situation it's clear that the answer is a woman. Yes, even if you imagine a boarding school for girls only and with female teachers only, "Who did it?!" is still translated as "Kto to zrobił?!". Even if it's a girl for sure, even if it's more than one person for sure.
So yes, "Kto będzie używał tej maszyny?" is equally correct, while "używała" would be wrong.
By the way, "co" is grammatically neuter singular. Just to add to the "kto" thing.