You can step back from being executor any time in the process. I would be very mad that the aunt I was “very close to” who I worried about being warm, and fed and took to multiple appointments, decided a cousin in the states got it all. If the expectation was 1/14th that was at max 43K, and probably with tax, estate costs, etc maybe 35K.
So it is definitely not worth challenging for that amount. Being an executor is thankless, so I would take TomEdisons advice and have a chat with the solicitor and he may give you more info on the context of how she made her decision, they make notes at the meetings and he may share some insights.
And then I personally would resign being the executor and walk away from it all. The house clearance, the sale, the paperwork, and so on. I would have no qualms letting the other cousins know that everything was left to the US cousin and his kids and that despite what everyone thought the aunt had plenty money in the bank for food, heating and transport.
Then just be glad you were in a position to help her while she was alive. Your presence was probably a great comfort to her and you were very charitable in how you treated her. Many older folks have a terrible fear of eating into their savings and would prefer to go cold and hungry.
Folks are odd. I know of a gent who died in early 60’s, always single, had a tiny house. On death left everything to a child he had fathered in late teens who had been adopted. Adopted child did have a minor relationship with birth father, acknowledged death and inheritance and as far as I know, the house stands there 10 years later crumbling to the ground. Something the child never wanted or knows what to do with. The siblings of the dead man have to field regular queries from people who would love to purchase the house or rent it, but no action has been taken by the child.