Disabling folder view is by design temporarily because KDE devs are revamping code. It should be back by 5.12. If this dev post is any hint, it means that distro maintainers are able to enable the folder view widget if they want to
It’s not possible to ship ~/.config/plasma-org.kde.plasma.desktop-appletsrc (either directly or through /etc/skel).
~/.config/plasma-org.kde.plasma.desktop-appletsrc is weird. It’s auto generated. A premade file won’t work. Then all files related to it would require to be pre-shipped. I.e. ship as many folder in home folder until this works, which is a very unclean solution. It has to be implemented using plasma scripting somehow.
I was also looking into changing the wallpaper depending on live or non-live mode.
To change it via the command line you can use the code posted here: Right click wallpaper changer. - Kubuntu Forums