My Mom passed away 12 days ago. Her dementia finally won.

I'm very sorry to hear about your loss. You've been through so much. At 91 your dad certainly has some dementia issues causing his detachment from the grieving process. My dads dementia was starting to worsen about 2 years ago when my brother died suddenly at age 62. I too was dismayed at dads behavior and lack of understanding what had happened. When the hospital ER called and told my folks that brother had been admitted in very serious condition dad refused to take mom to see her soon. She had to call her grandson to make the 30 minute drive to the hospital. They were able to see him a few minutes before he died of a ruptured aorta . Dad was oblivious to the situation. I live 600 miles away and got there the next day. Unwanted to choke dad at first but I realized that he was not processing this normally. I've yet to see him grieve over his oldest son. Maybe our situations are a bit different but please consider if this is the dementia causing your dads inappropriate behavior. Don't let this wreck you life. If you need to distance yourself from dad that's understandable and you shouldn't feel guilty. Do what you have to do to survive this tragic time.

I'm so sorry to hear of the loss of your precious Mom. You have been through so much and need time to go through the grieving process and I'm sure that's going to be tough with your thoughts focusing on your Dad. Its sounds like Dad has some issues with dementia himself. This would explain some of the things he says and does. With that said, do what you can to get through this period of time to help heal yourself and if that means you need to detached from Dad for awhile - then so be it. Eventually you'll need to "deal with Dad" but your whole health comes first.
You aren't alone in feeling the way you do. I have a parental situation similar to yours - except Dad has the dementia and he is also like a life-sucking vampire (MANY years before he was officially diagnosed). Mom is the buffer, and they both live in an assisted living community close to my younger sister. My sis and I work well together to watch over our parents because we have DPOA and take are of all their finances and bills. Dad's dementia is progressing rapidly and it won't be long (I give him 6 months or less) and we'll be putting Plan B into place. It will NOT be pleasant as Dad has become combative and physically aggressive (he's attacked my sister twice). Eventually he will need to be separated from Mom (for her own safety), but we have to do what's best for everyone and personal safety is paramount.
I understand that you want to take care of your Dad, but do you have any siblings or anyone who can help you in this matter. Please don't walk away from your business and home. Find out what resources are available in your area - and consider an assisted living community where he would be helped that is close by so you can visit him. Seek legal counseling as well to see if you can take over your Dad's affairs legally if he has diminished mental capacity. You won't regret it. All the best.