I do question why someone wouldn’t do everything they could to help
@PegasusReturns
How much of your questioning is directed to the father of the single mother, the DC's grandfather who has volunteered his WFH wife, claiming he must go to work and "just can't" help ?
I think I'd see if the 3 of you could work together to make it work for all of you
Well that sounds like a great idea.
Instead, OP's DH volunteered her help without consulting her
Who does that to a partner, in any sphere?
a partner who can pick up some of the slack
Sorry, lost track of who said this but my question is "how is her partner doing, picking up the slack then dumping it into his wife's lap, claiming he is too busy to hel but not even asking her first.
I would strongly encourage OP to avoid doing anything to risk her job and her independence. Imagine what he might expect her to do, if she did no longer have a job.