What kind of soil is it in? It is important for it to be in a loose, free-draining mix. It is normal for plants to droop a bit after repotting, as roots invariably break. Try to keep the humidity up around the plant. As far as the white marks, that looks like sunburn to me. The peace lily is a low light plant and the western part of your NW exposure is likely providing too much sun, especially at this time of year.

Yep, classic sunburn. Wilting could be recent repot if it is not fertilizer burn or overwatering.

A third for sunburn - technically photo-oxidation.

Al

Well, I have found that my peace lily likes a fair bit of water. I let it droop only very, very slightly and then water, but then it springs right back upright again.

Brown edging on the leaves of a peace lily are caused either by too much water or not enough water. If you say you let it dry out enough to wilt, then it could be you're letting it wilt too much before watering. The one picture you posted there is rather severe wilt. Don't let it wilt even a third of that much. The leaves should only be showing a hint of a sag and then it's time to water. Make sure your pot has drainage and water just enough to have water come dribbling out the bottom holes of the pot, let it drain fully, and dump out the water from the bottom tray. It doesn't like to sit in water.

Also, a peace lily does not want direct sunlight or really bright light. I actually have mine sitting in a corner away from the window and it blooms happily there. They say if the flowers turn green it is getting too much light. That said, it's not one for total darkness either. Off to the side of a north window is really good.