So, I expect that I should be able to use F-stop (premium v3.2.1) to create a folder on the external sd-card (which would be its own folder with write access), and then be able to edit images that are inside this folder.
Yes, we could add that ability in F-Stop but we chose not support this feature because if F-Stop is uninstalled for any reason, that folder is deleted by the system and you would lose EVERYTHING that is stored in there. Now, you can add this folder yourself using the default file browser or any other app that has write permissions but we do not support it in F-Stop.
If this is a global restriction in KitKat, then it should logically not depend on the device and the specific app under question. On the same device, MxPlayer seems to be able to make changes to external sd-card.
It is a global restriction in Kit Kat but we used a workaround that many other apps used to give some devices write access to their SD cards. It just so happens that this workaround does not work for all devices. I don’t know how MXPlayer managed to find another method for writing to the SD card but I sure would like to know.
I can tell you that we are working with the new APIs in Lollipop and have had some success in writing to the SD card. It certainly isn’t finished yet but it is promising as a path forward.