I eventually gave up trying to do this with i3. I was able to change wallpapers finally with Nitrogen, but it doesn’t provide slideshow functionality.
Xfce however works just fine with rotating backgrounds and to my surprise, allows for limited window transparency via the Window Manager Tweaks program, something I had been struggling to get working with i3.
Figured the transparency and rotating wallpaper features were worth the sacrifice, so I decided to go back to the defaults.