If I've got a project which is set at 1280x720, and I import a 1920x1080 video into it, and I zoom that video to 1.5 times the size with Crop, Scale, and Position, then I should get the video at its original recorded quality (because Kdenlive zoomed it down to fit it in the project, and then undoes that zoom again when I scale it back up). But I don't think I do. I think what happens is that Kdenlive zooms the video down to be smaller, and then zooms the scaled version back up again, thus losing a bunch of quality! How can I avoid this?

I have done similar zooming many times with "Pan and Zoom" effect without visible quality loss. Give it a try...