Super OCD. Perhaps Poirot and Countess Vera Rossakoff had more than a little admiration for each other...
The detective we see on screen is not PoirotWhoever he is, he refers to Poirot in the third person. Is it possible that Poirot is really like Mycroft Holmes, a figure hidden from the world and only in contact via an accomplice?
Poirot wears a fake mustache in Curtain because he lost the original in The Big FourAfter faking his death due to a explosion Poirot disguised himself as Achille in that disguise he had a scar in a location where it would impossible to grow a mustache as such he'd obviously have to wear a fake mustache for the rest of his life just like he did during the first part of his Achille act, that's why he had a fake mustache in Curtain.
Poirot was Nortan's final pawn in his stream of murders by proxy.When Poirot presented his belief that Nortan was murdering by proxy Nortan smirked and seemed unconcerned by Poirot's plan to execute him. Nortan was planning on corrupting the Great Hercule Poirot into a Murderer from the beginning!