Hi
@ji2216 - thanks for taking the time to post.
I'll be the first to admit I don't have much experience of caring for a PWD first-hand (my parents look after my grandma a couple of times a week), but a lot of what you're saying about your dad's lack of attachment to his family and combative nature rings very true in relation to Grandma and her relationship with my family, especially my dad (her son). She's also very bad for confabulations and accusing those that point out her lapses (again, mostly my parents), as well as accepting outside help. If you take a look in the "difficult feelings" forum, you can read my thread on that whole saga, which may help.
As for the other issues you mention - the neighbours are simple to deal with in theory, as from what I can tell they have good intentions in placating your dad. If you approach them as his carer and say frankly that you think what they're currently saying plays into his delusions, they're likely to understand.
The obsessive focus is a tougher one. Again, my grandma has this in spades, particularly about rent letters, her bank book, or individuals stealing from her. The go-to method, for my parents and doubtless other carers on here, is distraction. My parents usually do this verbally, but others I've seen on here tell them they'll get back to something after watching a bit of TV, or having a cuppa. It's never guaranteed to work, but sometimes knowing you have something in your arsenal makes the symptoms of this disease that much easier to deal with.
Stay strong,
TC x