Consider contacting the local office of Adult Protective Services. They may be able to advise you on how to address this. Also, talk with the social worker/s assigned to your father while in rehab. They have a vested interest in assuring that folks are discharged to a viable and safe situation.
Their motivation in part is driven by the rather high penalties they face when a person is readmitted again shortly after discharge. Means the discharge time or arrangements were not adequate, and yes it has to do with the reimbursement issues ACA enacted.
I faced a similar circumstance with my mom back when. We kept her in her own home as long as logistically possible, but eventually reached the "point of no return." In her case, I contacted the local Alzheimer's Assn. and they guided us through how to stage an intervention to get her into an assisted-living facility.
There comes a time when stairs become an insurmountable hurdle, with or without supervision and assistance. Handling the inevitable vulnerability and fear they experience is a big part of getting them moved. Losing their sense of control, coupled with their inability to visualize moving to a safer space motivates much of the resistance you're facing.
Sending prayers for a gentle, expedient transition for your father and mother. It's not easy.