the point of Clothilde (and Dynamics etc) is that you don’t have to have keyframes, they are engines which mimic real world physics to some degree based on their settings. The curtain stretching at first is just natural as the cloth engine lets the cloth relax into where it should be based on the cloth settings.
Why the curtain doesn’t open properly must have to do with the settings – Flexion is set at 50 but in spite of it’s name it’s a form of stiffness so it should be lower, even set at zero it’s not opening correctly so I suspect the structure of the curtain has to change but it will take some testing.