Thank you for sharing your story it's quite a shock my dad is very old school nothing is ever the matter and he never shares when things are not well with him so it's very difficult to see things like this creep up, I will speak to my mum when she's back about next steps and ask her to keep an eye on him
My grandad was similar. He has been gine 5 years now.
But looking back, before this started, he started to publicly complain about the cousins and one aunt borrowing money.
Which he had never complained about. Then when I visits his first words would be 'you haven't come to borrow money have you', even though I had never borrowed anything off him. Now, I can see how it started a good while before we picked up on it.
We just thought he had, had enough of them taking the piss. The aunt borrowed £500 from my nana and she died shortly afterwards. Grandad asked for the money, my aunt insisted she had given it back to my nana. But nana was housebound. The money was never found in the house. Even after we cleared it. My Grandad always felt she had used her own mothers death, to get out of paying it back.
And I think these things weighed on him more than the rest of us knew. So then when the dementia started, it was always about his money being stolen.
He would spend hours on the phone to banks, naval charities (he served in the navy), utility companies etc, convinced they were taking money from him or not giving him what he was entitled to.
It took us a long time to realise he wasn't just belligerent. I sometimes wonder if we could have helped more if we had picked up on it sooner.
I think as a starting point, you have a good plan.