In the spoken version of this sentence, is it only the tone of voice that identifies it as a question?
This is a good question. I was taught that to indicate a question in Swahili, you begin the sentence with "Je," but this course does not seem to use that.
A belated thanks.
How about, "mom are you lighting the fire?"
Then it would be unawasha. The a- prefix can only be used for third person, not second
Is 'a fire' acceptable instead of 'the fire'?
Yes; if it's not accepted, report it
Lighting a fire is acceptable for 'igniting'
Said "is mom setting a fire?" Is that really wrong?
Usually "setting" a fire refers to fires set deliberately by arsonists, or by firefighters, but not to start cooking fires.