@Ross at Play
I think the 'would' in your sentence is being used as the past tense of 'will'. To be consistent, you need either 'Ma wouldn't be coming home that day' or 'Ma won't be coming home today'.
Good point, but I'd originally chosen "wouldn't" to insinuate "maybe", rather than simply the past tense of WILL come home. I chose the word because he's not confident of anything, but is afraid of yet more bad news.
As I said, this is largely a problem because of my love of complex sentences (which places the narrator's interjection near David's spoken dialogue). If I stuck to simple sentences like so many others do, it wouldn't be an issue (note the use of "wouldn't" to denote 'not as likely to', rather than the simple past tense of 'WILL not'). That said, the sentence still works better as a combined whole, as the narrator's interjection explains David's hesitency, and the entire things is added as an attribution, rather than making it the central element of a stand-alone sentence.
This single sentence comes amid a long-simmering tension over the course of the book, and provides a key clue in figuring how to proceed (which Billy is about to relate in describing WHY Ma isn't coming home today).