aha, so the problem seems to be that when loaded via plasmoidviewer

m_package.metadata().pluginName() == "org.kde.desktopcontainment"

when using a custom interceptor to circumvent it, i still get strange errors. can someone help?

QML debugging is enabled. Only use this in a safe environment.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Plasma/FocusFrameStyle.qml: File not found
file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Base/ButtonStyle.qml:153:31: QML Item: Binding loop detected for property "implicitWidth"
file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Base/ButtonStyle.qml:153:31: QML Item: Binding loop detected for property "implicitWidth"
file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Base/ButtonStyle.qml:153:31: QML Item: Binding loop detected for property "implicitWidth"
file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Base/ButtonStyle.qml:153:31: QML Item: Binding loop detected for property "implicitWidth"
file:///usr/lib/qt/qml/QtQuick/Controls/Styles/Base/ButtonStyle.qml:153:31: QML Item: Binding loop detected for property "implicitWidth"
qml: View QML loaded
kf5.kservice.sycoca: Trying to open ksycoca from "/home/phil/.cache/ksycoca5"
Trying to use rootObject before initialization is completed, whilst using setInitializationDelayed. Forcing completion
Plasmoidviewer detected: circumventing security  QUrl( "file:///home/phil/Dev/KDE/steam-plasmoid/build/org/kde/plasma/private/steam/qmldir" )
Plasmoidviewer detected: circumventing security  QUrl( "file:///home/phil/Dev/KDE/steam-plasmoid/build/org/kde/plasma/private/steam/qmldir" )
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
qml: New Containment: ContainmentInterface(0x194b390)
QQmlComponent: Component is not ready
QCoreApplication::postEvent: Unexpected null receiver
"file:///usr/share/plasma/shells/org.kde.plasma.plasmoidviewershell/contents/applet/AppletError.qml"
 "Error loading QML file.
File not found
"
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() instead of kservice_desktop_to_json() in your CMake code.