@willow8 You will never be able to make him understand that he needs help with stuff. All you can do is subtly offer your help in a circumstance. Get used to grouchy, self deprecating resistance to so many things... It is tricky to transition to getting them to allow outsiders to repair things that they cannot repair any longer, just try to ease into the positive ways that someone who has professional knowledge could get it done in a shorter period of time, and maybe age plays into the repair (not dementia) and that this particular repair is more dangerous than other repairs...
The anger is his frustration with the disease and his lack of the understanding that he used to enjoy. You will have to grow thicker skin, unfortunately, and just try to not engage with him when he's yelling at you, (easier said than done).
I don't know what to tell you about the 3 hour drive he wants to do alone. My husband still drives, also, and gets confused sometimes. He knows he can't drive long distance or on freeways because we've eased him into that knowledge over some time. I'm really uncomfortable with him driving, as it is a dangerous liability, and one that I am dreading to delve into. His car is suddenly on the fritz this weekend and I'm thinking that this could be a Godsend, but we'll see.
That's my 2 cents for the day. I wish you luck and peace.