He says there are “divorced and remarried (or cohabiting) persons who have the intention to change their condition but cannot."
'Cannot'?
'Cannot' cease sexual relations with this person they are living with outside of the bonds of Holy Matrimony?
Are we speaking of people her who suffer from global cognitive deficit (mental retardation)? Breaks from reality (schizophrenic/psychotic)?
Or are we speaking of people who are being held by force, against their will, in some form of involuntary servitude or bondage (an abducted person living in sexual slavery)?
Those would be persons subjectively incapable of mortal sin because of severe internal disability or external coercion.
But if we're talking about a person with normal cognitive processes and a normal degree of control of their own conduct, I'm not quite sure how it would be literally impossible for them to cease committing adultery.