Issues with SystemdProcessRunner and environment variables

However, SystemdProcessRunner currently has issues with environment variables:

If neither KProcessRunner::modifyEnv nor KProcess::setEnv is called when initializing the KProcessRunner, our environment variables are not propagated to the new process as the new process is spawned by systemd --user, which breaks e.g. inheriting XDG_ACTIVATION_TOKEN when the kio client does not have an active window.
If at least one of those are called, our environment variables are propagated as these two functions initializes m_process->processEnvironment() to QProcessEnvironment::systemEnvironment() and SystemdProcessRunner pass it to systemd. However, this fails when our environment variables include variables that can not be passed in this way to systemd, e.g. when the name of one of the environment variables contains .. This can be demonstrated by e.g. running env "xxx.yyy=zzz" dolphin and trying to open a file or running env "xxx.yyy=zzz" kioclient exec file:/usr/share/applications/net.minetest.minetest.desktop when minetest is installed. Note that e.g. Pulseaudio uses PULSE_PROP_application.name as an environment variable name.

Given those, here are my questions:

Do we revert 24977bfc as SystemdProcessRunner was not actually the default on Plasma and does not seem to be ready for prime time yet?
Do we want SystemdProcessRunner to propagate environment variables from parent? If we do, how do we filter out environment variables that systemd does not like? If we don't, should we special case XDG_ACTIVATION_TOKEN like the DBusActivationRunner did?