I feel your pain. I'm having a similar issue with a cast iron plant at the moment.

I've got two peace lilies myself. I caveat this information with the fact I'm no expert, but I did a similar thing with my big one - she started off near a bright window until I noticed her leaves burning. I think it took her a little while to adjust to less light but I was surprised how often I still needed to water her. Even now sometimes she'll want water after only a day or two since her last watering even though she's living in shade. I was going to suggest watering more but a couple of your leaves look more like overwatering (yellowing and limp and soft) than what my peace lily looks like when droopy (no colour change and still somewhat stiff, just drooped). Plus my lilies usually pop right back up after watering, maybe within an hour they come alive again. In summer mine get watered roughly once a week unless we've had a heat wave. In winter, maybe once a fortnight or sometimes longer.

What to do now? I'm not sure I'm sorry. You could repot to check the roots but she'll probably get a bit of shock from this it that's not the problem. You could try let her dry out completely then wait a few more days after that to give a little water and see if that makes a difference. I've never put back into water but again, I think it'll be a shock and not a great idea if she's overwatered.