The OP's situation is totally different to having chosen to be a SAHP:
She has been deprived by accident / illness of the job, probably profession, that she used to do.
She is "trapped" into lifelong SAHM work that she probably shared before with her OH or with a paid cleaner
and would only be human resenting the frustration of this life sentence without parole
She desperately needs to be able to do something else as well,
but is probably NOT capable of the hours & reliability required of paid empoyment
She brings in a "salary" from the insurance company, so her OH is NOT the sole breadwinner
he doesn't carry that responsibility.
The insurance company WANT her to volunteer, to help her mental health & coping - that may even be a moral obligation to them, to avoid becoming even worse and possibly then becoming entitled to higher payments.
The OP's disability does not mean the OH is entitled to demand her wifely "duties" be fulltime - wtf ! - so that he can have an easier life than before.
She certainly has a worse life now
Both parties are entitled to the same amount of free time for themselves and it sounds like it is only some days that the OH - horror - has to put his penis down and parent his DC.
The OP needs her volunteering as therapy, a small way to make her disability less frustrating.
Her OH should understand this.
Would it be more acceptable if she went to counselling sessions instead, a couple of nights per week?
Volunterring is often more effective and anyway counselling is usually non-existent even for chronic need.