When you repotted, did you leave the root ball and old soil intact, then fill in around the edges with new soil? I think that's what you're seeing with the soil separation.
I also suspect that at some point the soil dried out so much that it no longer soaks up water very well, and water runs right through it without really getting it wet. The cure for that is to submerge the pot and soil in water, and let it soak for several hours or even overnight.
I would:
Add more soil to level out the pot.
Trim off the leaves that are turning black.
Soak the pot in a bucket for several hours.
Water more--Peace Lilies are streamside plants that like to stay moist. Don't wait for it to wilt.
Bowls of water actually do very little to raise humidity, I'd skip that and just keep it well-watered.
Feed with a complete fertilizer, diluted to half the recommended concentration. Repeat every month through the summer, stop in the cooler and darker months, resume in Spring. (I use Miracle Grow All Purpose, or leftover orchid food, or Miracid, or pretty much anything, very dilute.)
Maybe give it a little more light, no direct sunlight and not outdoors unless you have a place with deep shade all day.
If it doesn't perk up, I'd consider repotting next Spring. Use a rich houseplant mix, I've had good luck with Miracle Grow Indoor Potting Mix.
(I'm not endorsing Miracle Grow, just sharing what works for me and is readily available here. I'm sure there are plenty of other good choices!)