CMOT, I found Oliver James' Contented Demential book flawed, but there were a few really useful ideas. One was the idea of deciding in advance the "tipping point". At what stage does it become too much. As in the toll on you and the people around you.
Your dad appears to be in a tunnel of clinging on. The sad thing is that this must be really stressful for both of them and that they have probably missed the boat for a setting like very sheltered which would have enabled them to simplify their lives and so cling on longer and in a less stressful way.
It is only going to get worse. I think my dad hid my mums memory loss so he could die in his sleep in his own bed, with me to pick up the pieces. Your dad instead sounds as if he might like this solution but is instead heading for a slow decline, with stress and anxiety contributing.
Are you or health professionals aware of another setting that might work for both of them. With your dad perhas offered that this is for your mum and that when she dies or is effectively bedridden, he can return home or to something more suitable.
And what is your tipping point. The point at which you decide that you cant offer any more. The toll on your health and your family's well being is too great. Can you explain to your dad that his firends have dropped away because they can't cope, and that you are getting close to that stage.
That all sounds a bit brutal, but I hope I am forgiven. The anonimity of the internet. flowers