Overall looks good to me, I also now understand why you don't use appScope as part of the unique id, hovewer I've been suggested by Alex to use flatpak_installation_get_display_name() or flatpak_installation_get_id(), which would result into a more readable unique id.

In D5510#103509, @jgrulich wrote:

Overall looks good to me, I also now understand why you don't use appScope as part of the unique id, hovewer I've been suggested by Alex to use flatpak_installation_get_display_name() or flatpak_installation_get_id(), which would result into a more readable unique id.

I wanted to do that, but they don't seem very reliable. See:
qDebug() << "ggggg" << flatpakInstallation << g_file_get_path(file) << "." << flatpak_installation_get_id(flatpakInstallation) << "." << flatpak_installation_get_display_name(flatpakInstallation);

$ plasma-discover --backends flatpak-backend |& grep ggg | sort -u
discover(3265)/(default) FlatpakResource::installationPath: ggggg 0x7f5170009160 /var/lib/flatpak . default . Default system directory
discover(3265)/(default) FlatpakResource::installationPath: ggggg 0x7f51700092b0 /home/apol/.local/share/flatpak .  . 

These identifiers don't seem to be very reliable, at least on my system. Using _get_id was actually my first approach.