In my manger scene, my mother insists on not putting the baby Jesus in until Christmas. Many have told me this is proper. But then why is Mary kneeling at an empty feeding trough?

Many families do wait until Christmas Eve or Christmas Day to put the baby Jesus into the manger scene, for the reason that he has not yet been born, and that an empty manger captures the spirit of Advent. This, as you’ve pointed out, means that Mary ends up spending a few weeks kneeling by an empty manger. If manger scenes had poseable figurines, I suppose that could be changed, and she could be in some other posture. In general, though, we just work with the limitations of the figurines we have, and try not get too literal about what it means. The important thing is that Mary, like all of the rest of us, was actively waiting for the birth of her son, and celebrated his birth when he did arrive.
By the way, many families put Jesus in there as soon as they put up the crèche, which is fine, too. It’s really a matter of personal preference.