I tried a bunch of aplets and observed that the configs were properly restored on opening the dialog, switching between config pages, the apply button worked properly, and the settings were applied correctly. Both in applet and wallpaper configs.
I noticed that switching between Image and Slideshow wallpapers breaks sizing of the images and everything but I think this has been the case previously, too, most likely because the wallpaper plugin checks for currentWallpaper being slideshow, so during the switch animation the old wallpaper config is still there and notices it's now slideshow and then shit hits the fan. Not sure if we actually support the usecase of if (configDialog.currentWallpaper === "foo"), we should make that a property on the config page like cfg_ rather than having it access stuff outside but that's another patch to be done :)