And when the crisis comes you may well discover that things were worse than you realised and there will be waves of real anger as you know it did not need to be like that.
What do you want to happen?
My guess is that your mother does not want to move your father. It would in some way be a betrayal of their dream, and a further bereavement. And that you are looking for two solutions. First to help your mother manage whilst your father is alive and then what to do after he has passed away.
My mother, who was 80 at the time, somehow held it together through my dad's terminal cancer, but then had both a major health scare and a marked deterioration of her cognitive functioning. If tiring for us, caring for the elderly infirm must be exhausting for their partners. If you are just clinging on, it is very hard to take a more strategic view.
However eventually, as you say, something will happen which will make it impossible to go on.
There won't be that many local options, so there may be advantage in exploring them now, to enable you to move quickly when your mum starts to waver or the crisis hits. The health visitor or other professionals should be able to provide a steer towards sensible alternatives. You then may need to consider whether you need to sell the farm to buy something else. Or are you and your brother able to access funding. (We ended up extending our mortgage in the short term to be able to provide bridging finance.)
To be honest I was completely unable to have the "what next" conversation with my mum. However if my dad had not had a fatal stroke I think my mother was getting close to a point when she would have been willing to consider respite or hospice care for my dad.
In the short term is there any scope for tenants? A young couple from the village perhaps. Rural accommodation is always hard to find. An agreed schedule of labour instead of rent? Though like au pairs the right person is wonderful, the wrong one a nightmare.
After my father died my mother was still unable to give up their dream flat, even though it meant she was very isolated. I wish I had worked harder to look at alternatives with her. One advantage of eventually buying a sheltered flat was that there was a level of marketing aimed at her. She was invited for lunch with a group of other residents, shown round by staff, and the experienced manager was able to talk through the benefits. Each time she went she left enthused. (Then, obviously, she forgot!) If the next step is likely to be something private then worth visiting early. Half the sales work will be done for you, she will be able to see the alternative on a practical level. Who knows, she may then see that warden support etc, would allow your dad to stay with her longer, rather than having to go to a hospice or hospital.