Phenomenal review, Nico!
Haute Tension was my baptism into the French Extremist genre, and I loved it. I can understand the detractors, I can especially understand the people who thought this was a rip-off of the Dean Koontz book Velocity (because the setup is almost identical), and once you know the twist it's sort of like watching Fight Club and trying to figure out how everything worked.
I think your take is perfect though: Marie's an unreliable narrator, and we're only let it on the 'truth' a couple of times throughout the movie. Everything else we see that's not, say, the security camera footage from the gas station is all viewed through the lens of her telling us what happened, and she's either lying her ass off or she's rearranged reality in her head to correspond with what she wanted to happen. We can't make sense of it because she can't make sense of it.
Makes sense to me... :D
Now, for me, the real test of a gore-hound is Martyrs. I've not had a film disturb me that badly since the first time I watched Cannibal Holocaust (and even then, the disturbing part of that film was the animal cruelty), but Martyrs screwed with my cerebrum for the better part of a month. :)
Mine too! A friend and I started a series of "French Horror Movie Nights" with this one, and it really stuck with me. I can understand the complaints, but I ended up being pretty set on its brilliance, and I'm eager to rewatch it with the knowledge that I now have and the contemplating that has been done since we first saw it. The only problem now is treading carefully on that line between "enjoyably messed-up" and "traumatized for life" as we wade through the better part of the gore catalog.