Client won't download updates unless app is restarted

It shouldn’t matter if the image is set as a background or in full screen region - it would matter on Windows clients, but on Android it should be fine either way.
One thing that you may have a look at is the dpi of your images, Displaying Images
really high dpi can cause issues in some cases.

Regarding changing the image, yes you’re correct, that will replace the image in CMS library and on all layouts this previous image was assigned to.

General advice, other than keeping the durations in mind, not really, there might be some case specific recommendations/issues, but that’s more on a case by case basis.