Is she or isn't she? Don's position seems to rest on the idea that Maigret has been taken in, even if partially, at some point; and what is more that the closing scene suggests his feeling for the woman. It's like Bogie at the end of The Maltese Falcon. However, it's all so tentative, and the plot details are so understated or not stated at all, that the description "marginal" as opposed to full-fledged does seem to apply -- but see below where I argue to the opposite conclusion.
My own opinion is that Pierre Renoir does a fair bit of smiling at her attentions and allows her to be seductive, but we cannot rule out that his wiliness is at work with her as with everyone else. He could be simply shaking his head about her at the conclusion. He's had an experience, for sure, but all the time he was still being detective. The closest we get to his real feeling is when he unmasks her and he's actually quite angry to have to do it, and he kind of rues what he has found out or surmised about her. He wishes it were not so. Taken with all his other interactions with her, it's clear that he is not objective to her. He rather was smitten at the outset when she first came on to him. She was touching him quite intimately. A woman's touch can be amazingly seductive.
Dan's view is agreement with the critic who calls the character too "marginal" as a femme fatale, or not full-fledged.
How can this ever be settled when we have no measure of marginal vs. full-fledged? We have that critic seeing merely an echo of the man's anxiety and failure, and we have Don seeing Maigret waiting for 2 years, hoping for some rehab and an eventual tryst. What I see in him is that he's rueful. He'd like to have her, it's something of a shame he feels that she's such a whor* and has been so deceptive, but I think he actually is amazed that she as a woman in particular or women in general can behave as she has. He's learned something about her or maybe female nature. Actually, as a cop, he already seen all sorts of characters, so that what he learns is more about himself and his vulnerability plus about her as a unique person. She's not at all who she seems to be. That take on what he communicates in the end tends to support the femme fatale notion.
I would not rest a positive case for her being a femme fatale on having taken Maigret in. I'd rest it on her having tried to do so, as Mary Astor does, on combining qualities that are both attractive and repulsive. I would rest the case on who she is, what kind of person she is and has been. He's affected, but we're all affected by people we meet and have some doings with. I think the debate hinges on what sort of person she is or has been. I think she's more than an echo of men's anxiety and failure, as that critic argues. She's a certain kind of character in her own right, and that character is one who uses wiles and deception, including seduction.
Add it all up, and she's a femme fatale. Not simply marginal but the movie we have before us leaves so much out about the case's solution that her role is tarred with the same "marginal" brush, and that's not 100% fair. She is marginal in that sense of not being a character who's fully fleshed out. Her back story is dumped on us without preparation. I therefore see the merit in both sides of the debate.
I should look to the place of this character and that of the femme fatale in Under Secret Orders in film history, i.e., their influence.
I can't speak for the French end of it. The American end should be clear by now. Is it? Is there a full-fledged femme fatale prior to Brigit in The Maltese Falcon? I think John Huston is a major, major figure in the development. His hand and work make a quantum leap. Was he influenced by La Nuit or by figures in the spy noirs? I have no idea, but I do see some resemblance of Brigit and Bogie to Maigret and Else Andersen. I can't comment on Under Secret Orders, not having seen it.