1. Editing sources
Python-apt-common-devuan is not automaticly installed when choosing to use KDE. With Xfce it is. I know it says it's for aptitude and synaptic, but software-properties-qt also needs that package to work. Both Apper and Discover uses s-p-qt for editing sources.
After installing that package, there is still problems with the Devuan.info that it has. All the underscores in the description lines causes s-p-qt not reading those lines. Then there is something wrong with the MatchURI. If it's the regex version, when you click the components in the first tab. It will not tick those boxes, instead it will make the entry to the other software tab. If you change the MatchURI to deb.devuan.org everything works corretly.
Also the codename in the /etc/os-release needs to be only the version you are using for s-p-qt to run.
Although after installing the package and fixing the above problems. Still can't open to edit sources with Discover. Opening Discover from terminal and then trying to open software sources, it says in terminal, could not start /usr/share/applications/software-properties-qt.desktop.
And lastly when you get the s-p-qt running, it says Ubuntu Software as the first tab.

2. Layout
The selected keyboard layout at install isn't saved/used with KDE. So you need to set the layout again.

3. Apper
With Apper , I can't install anything. Failed to obtain authentication is given as a reason. Need to start with sudo to use it.

4.
Why is policykit-1-gnome installed in kde system. The package description says "The remaining users of this implementation are Cinnamon, XFCE and Unity."

Will add if I come across anything else.