Do you have recommendations for similar movies?

No nnmko you are not pretentious at all, you seem to be a very keen an smart viewer. I'm just sorry my language skills are not that good (I'm from Holland) and my analyzing abilities are not that clear as yours. You describe the styles very well and now I understand things a little better. (The reason I go to this IMDb-Forum)

Unfortunately Polygram company doesn't exist no longer - I saw a documentary about it. They were in the end a strong European movie-company producing interesting films for wider audiences next to experimental and arthouse films. (occasional hits like "Four Wedings and a Funeral" allowed financing other projects and caught Hollywood's attention; Jodie Foster went there to make Nell for example after failing to raise her project in Hollywood). You know, films that would fit well in Robert Redfords Sundance Festival.
But when the new Philps director didn't want to invest any longer and when they wanted to compete in the US and also moved there, they soon failed and went bankrupt.
I'm just interested in good movies and not at all in business and production, but the documentary ended in suggesting that the ten years or so Polygram was active (and got acclaim in Cannes film festival) may have changed (still dominant) Hollywood to shift a little and also take on interesting (non-commercial) projects.

I enjoyed the new vague of mosaic-like storytelling form like in Amores Perros, 21 Grams and Babel, Crash and Traffic. I picked up "The Dead Girl" and it uses that frame also (I'm not sure if I could recommend that one because it uses murder and cime as main element next to ppl and situations and not the transformation or psychological approach). But those projects use the opposite of what you asked for: one situation, different ppl (and not one person, different situations). They both show what effect situations have on ppl.

I like the fact that you mentioned books. I never understand actors/directors telling that stage has that fourth wall or glass wall or so (we all agree we let you make beleive us and you allow us to pretend or so) and cinema hasn't. I seem always to be watching a script develop (and decide I want to go with it or not) I mean. Good writing, great idea and solid acting - a drama that makes some sense. You might like "The World According To Garp" by Irving I think the writer is called. That is a joy to read with stories within stories and parallel lives. A movie was made with Robin Williams and Glen Close and I thought it was good as well. I can recommend that one (although it is a story over a long period of time).

Some actresses in scenes of films with depth and solid drama are just iconic to me. Glen Close at the end in "Les Liasons Dangereuses" expressing defeat and shame in a close-up, Kirsten Scott Thomas throughout Il y a longtemps and Naomi Watts (also in 21 grams) in David Lynch' "Mulholland Drive" the audition scene and later in an extatic psychotic moment. Very hard to watch and I'm not sure if I could recommend those titles cos it deals with emotional abuse.

One I can recommend (although also with murder and crime) is with Ryan Gosling (he seems to be in a lot of interesting projects) who was also in Half Nelson is "The United States of Leland". His character sort of loses it and then finds himself in different situations (like you're interested in). He was in "Stay" too revealing the fantasy-state he is in or psychotic mindset he has since having to need to escape psychologically after a tragic incident he was in. So in a way that doesn't count as an A to your Q in OP because like in "The Usual Suspects" he just makes up a story and which is not the actual situation. In both films they pretend to be someone else.
Not very happy films (death, dark) so I end with a touching one: "Snow Cake": a mentally challenged woman who needs her situations to be the same at all times now having to deal with the fact that her daughter is dead. The short time that there is, Alan Rickman helps Sigourney Weaver with that. Maybe choose this one and Garp over the other titles.

In the (as you put it) "long tradition of character transformation through action (which I also find very interesting, provided that the relationship action/transformation is not a shallow one)", please allow me to plug my favorite director Sidney Lumet and "The Verdict"? It has rythm and balance and Paul Newman and close-ups only where it matters. Having an addiction AND having a conscience (like in Half Nelson).

Yes that is correct: "Betty Blue" is "37°2 le matin" from 1986. Something goes wrong in sexy Beatrice Dalle's head (becomes another person gradually in situations) and her boyfriend Jean-Hugues Anglade stays with her. Also french and nostalgia for me is "Le Grand Bleu" (1988, Luc Besson) with Rosanna Arquette. Very poetic and strange: her boyfriend prefers to stay under water with the dolphins over connecting with her. How weird a situation is that? Romantic tragedy dramas, maybe not what you meant, because in both cases the perception of situation (reality) changes for one person and over a longer period of time.

So LOL it turned out to be quite a challenge finding matches to your "riddle": same person/different situations/SAME TIME (or short period of time). Maybe Snow Cake comes close, but... the situation is only set in and around her house. So you don't mean transformation/evolution/adjustment (different faces) but one person/one face/one mindset in different situations. I'll add your two french titles to my DVD-collection and I will let you know if ever I find a better match in my collection!

Cheers.