It is a Poirot and is of that world: upper class folk with money, and those who serve that world or engage with it; this is a pleasant environment, there is no grime. And everyone lives near each other and the traffic is never bad; when Poirot asks a maid to come and visit him, she can arrive within ten minutes, London in those days was a small place it seems.
Now on to spoilers.
I managed to guess the central "swap", and once you've done that things become fairly clear. Since I was reading this over several sessions and not paying close attention I missed several minor details, having forgotten who Mrs van Deusen was for example. I also guessed the (s)he clue: and now I've seen that, I know that if ever you're in a detective story and someone sends you a telegram copy of a letter, rather than the actual thing, that means that inevitably there is a physical clue on the real thing.
Doubts: Lord E was cleanly killed by a knife to the back of the neck into the head. This doesn't sound like an easy kill; in a "confession" the killer says that she talked about it on-set with <medical person> sometime; but I really don't buy that. Nor is there any discussion, which I think you'd expect, about how the killer got that close; they were not an affectionate married pair. Also, our double is offered $10k to impersonate "for a bet". $10k is represented as a life-changing amount and therefore she jumped at it; but it should also have been such a large amount that she should have had very strong suspicions about what was going on.
Oh yeah and the bit about Lord E having a troubled marriage, and being somewhat odd, and having a rather attractive butler, would be said somewhat differently nowadays I suppose.