Condolences to you and your dad. January is very very recent.
As you and your father have probably been told, its very common for people to die when relatives are out of the room. I was told this can be simply because people can choose when to go to a degree in an end of life state and some want to do so alone either because that's their personality (just as some people when ill want relatives around and others just want to be left alone and lie in silence) or because they want to spare their relatives. He really shouldn't blame himself and I know that's easy to say.
To give you a bit of perspective, I've just spent time on a geriatric ward where I saw someone who should have had a limb amputated (But they wouldnt operate because they were going to die anyway) have the limb go black and basically die from poison/gangrene. People can die unable to swallow and making horrific noises choking on their own secretions. The suffering in death can be inhumane. I'm not suggesting that you necessarily tell your father this word for word, but to die at home with loved ones near is a huge huge blessing and slipping away unconscious is what we would all aspire to if you have seen how horrific death can be.
It could be grief - apparently that if you have a life long marriage, if one partner dies, the surviving partners chance of dying in the next 12 months increases by 50%. It is possible to die of a broken heart. He will obviously be devastated and I think all you can do is help him through his grief as best you can but you are going to be grieving too.
Practical things
- if is still living in their home, get him to move into her place -sit in her chair, sleep on her side of the bed. That way you aren't looking at the empty space constantly.
- persist in encouraging him to go to grief counselling. maybe put his name down anyway - Cruse where I am had a months and months long waiting list but it was honestly one of the best things I did. They said to me you should wait a few months anyway before you start to allow emotions to settle.
- as far as is practical, try to make sure he's not alone for prolonged periods. If it's an option, move him into your home for a bit or go to stay with him for a while or if you have siblings or other relatives this would work with suggest that to them or set up a rota.
The fact he is a shadow of himself and is slurring - as you said my first thought was a stroke but maybe it could be first signs of dementia possibly accelerated by the trauma of loss. I would refer him to GP for slurring and awareness of what is happening as it could be dementia related or an undetected stroke/heart attack.
Beyond that he is probably just deeply depressed and in the passionate sadness that grief is having lost a life partner. He probably feels what is the point without her. If he can get through 18 months, he'll probably come out the other side - that is the challenge.
Bereavement is a grotesque hell. We all want to turn the clock back.
best wishes to you both.