I use fall leaves for mulch everywhere. Just don't let it get real close to the base of the tree as it can add to rot to the bark and also allow critters cover to chew on the bark, both not good to the tree.
Soil that has an odor to it because of it being to wet can be dried out and re-used. I would use it mixed with some fresh to give it a new life. If you use the half barrel again make sure to drill lots of drainage in it.
Sandy

Thank you very much, Sandy. Your answers are very positive for me. I'll make sure the leaves don't touch the trees and to mix in some fresh potting soil prior to re-using of old soil.

Xuan