DanZZZen, here are a couple of links that might be useful for you if you are doing some reading.
alzheimers.org.uk
www.dementiauk.org/what-we-do/admiral-nursing-direct
Alzheimers Society is a great source of information and support about all kinds of dementia. Admiral Nurses are a (truly wonderful) organisation, UK based but they have a phoneservice where you can ring and ask for advice.
As I said before, I am really not wishing this onto you but your description of your dad rang so many bells with me that I do think it's worth looking into a little more.
Mine too was a highly intelligent man (still is) with a senior position in a very hierarchical organisation. He was accustomed to having his instructions obeyed without question. Hated being organised by anyone else. Still does.
With hindsight we now realise that as dementia began to take hold so he began to distance himself from family events (doesn't 'do' any other sort of social gathering anyway). He made up an excuse to boycott my sister's wedding (she was crushed) and cancelled holidays at the last minute with my mum etc.
As we learnt more about dementia my Admiral Nurse explained that people use techniques like this to avoid being in company with people who might spot that they are not as alert/in control as they used to be.
Dad was fantastic at this. Also at convincing health and social care professionals that he was fine. After a fall he was assessed by a social worker and completely pulled the wool over his eyes. He is so used to command and many kind, gentle people are used to accepting authority so are not always ready to challenge. He blinded them with language, big words, slighly patronising attitude towards them...
As Buda says, paranoia can play a big part. Dad has 'taken picks' against people all his life but with dementia this became even more pronounced. My 10 yr old nephew suffered badly, Dad's neighbour (stealing his post), his parish priest (lying about things happening in the church), my husband (sabotaging his relationship with a local organisation) etc etc.
Hard to know what to do with him being so far away from everyone. FWIW I don't think he'll miss the opportunity to see you and if you do go it could be a good chance to chat with his neighbours and get a more immediate idea of how he's doing. I'd try not to stay with him though, give him (and you) space to hide.
I do hope things work out.