I think this is for your MIL to decide. If she’s at her wit’s end memory care would be good just to give her a break. Especially now that he is declining. They will take over all his care: feeding, bathing, grooming, incontinence, etc. MIL can just visit and get a break. The staff would also be trained in caring for someone with cognitive issues. Assuming they have assets this would be private pay.
If she wants to keep him at home there are doctors that can do house calls like Inova Home Health — pretty sure they are covered by Medicare. At the very least he could get skilled nursing and someone to prescribe medication to help with the insomnia, etc. You could also hire unskilled private duty aides to help MIL for longer shifts.
Finally not to be grim but home hospice is also an option that will provide additional palliative care at home, but of course that also means forgoing some treatments. Medicare will cover it but you have to make sure everyone is on board before doing it.
Best of luck.