To be honest, this doesn't sound like a mystery disease to me, but it is nonetheless very disturbing for you to process and understand.
This does not sound like a dialysis problem, at least not in the first instance, but sounds more like that he has - sadly - widespread small blood vessel disease in his brain and that he is having (or has had) a number of mini-strokes - often referred to as TIA's (Transient = come on, then resolve or improve ... Ischaemic = reduction in blood supply ... Attacks = episodes of) ... ie: recurrent episodes where the blood flow to his brain tissue is either being reduced, or interrupted.
Diabetes +/- high blood pressure damage small blood vessels ... but ALL OVER the body. This occurs to, among others areas: the kidneys (kidney failure); the heart (poor heart function and/or multiple small or large 'heart attacks" = infarction; the eyes (poor vision); the nerves (especially but not restricted to the small nerves of the extremities - especially the legs and feet - with numbness and changes in sensation; and the brain (with multiple small stokes in patch areas throughout the brain that lead to functional [speech, movement, etc] or to memory loss).
The brain changes that result from this extensive (and sadly, in 2017, irreversible, small vessel disease) are often referred to as multi-infact dementia ... or a loss of brain function as a result of many small areas of the brain losing their blood supply. Some recovery can occur - especially early on - but as injury piles up on injury, the cumulative effect is a dulling or a loss of key brain functions ... speech and memory being two of the more visible to relatives and family.
Dialysis - especially if is too aggressive with fluid removal - can de-stablise the blood pressure which, in turn, reduces blood flow to the brain and can precipitate (bring on) new 'attacks'.
Is there a remedy? I wish I could say there was ... but from your description, from very far away, and without knowing all the details of your dad's long illness, this sounds very much what the problem is likely to be.
You need to sit down with his treating doctor(s) and ask them to explain, in very simple terms, the likely cause - though I fear I may be right - and to let you know if they think there is any remedy. But, again, I suspect there is not.
It is very sad for you - and very confronting for the family, considering all he has likely gone through, but this is - I fear - the truth.