I realise also it's become a Mn cliché, but parts of this the fussing about arrangements, the repetitive comments, the visible irritation and bafflement at a conversational topic that doesn't fit his (niche) interests remind me strongly of my own father, who is in his 70s, but whom I have long suspected is not NT. And definitely not age-related in my father's case -- he was this way when I was a small child.
But vent away -- I hear you. The key frustration with going out somewhere with my dad is his all-consuming parking obsession before events. He is absolutely obsessed with whether he will be able to find a parking place, and will often do a test run, or several, to scope out the parking (even when this involves a meal out a ten-minute drive from home), and decide, often arbitrarily, on a new pet parking spot, often a long way from the venue, but to which he will remain fanatically loyal, despite its inconvenience.
He will tell everyone about this parking spot. He will insist that it is convenient for everywhere, in the teeth of the evidence, and pride himself on his unique savviness on having found it. He pooh-poohs rival parking spots. If my mother has been in the city centre and he's collecting her, he also has a pet 'pick-up' spot which is nowhere near anywhere she is likely to be, and is in fact almost halfway home to my parents' house, but which suits him for some reason. (He will also tell everyone about this.)
Once I phoned him in a minor emergency, because our car had broken down, and I had toddler DS with me and no coats or pushchair, and he said he'd come and pick us up, but at his pet pick-up spot (which was a good mile and a half away from where we were, in the rain, across the city), and was completely baffled when I said, 'No, I need you to get us from where we are.' When he arrived, he had the air of someone heroically venturing into uncharted jungle, despite the fact that he worked a few minutes away till his retirement.
Years ago, I bought him as a present a short evening course at the local university on researching your family history, something he'd often expressed an interest in doing. A friend of mine, thinking of doing the course herself, asked him about it -- cue a fifteen-minute monologue which did not in fact mention the course at all, but focused entirely on the parking at the venue, and how you had to arrive after the day staff had gone home but before students and staff on night courses arrived to maximise your chances of a space in his favourite part of the car park, and his cunning in discovering this.
I really love my father, but he's exhausting, and the behaviours and obsessions are becoming more entrenched with age.
That was cathartic.
Vent away, OP. 