The leaves will just add nutrients as they degrade & potentially make your bog a bit more acidic (though with peat in there it probably won't). If you want to grow plants that like low nutrient bogs (like carnivorous plants) I think you would want to avoid the leaves. Otherwise they should just make for happier plants.

Calla lilies will be annuals in your zone I believe but I think they do pretty well in shade. I know hostas do but I don't think all of them like to have their roots really wet so you will have to check on varieties. Mint does well in the shade too but can be invasive so you will have to pull a bunch out occasionally.

Thanks kate rose!

Hosta do wonderful as long as you don't drown the crowns. I've grown more varieties than you can count and none suffered from wet feet.

Thanks wilddog.