I don’t have the setup to recreate your issue so I’ll take a couple more shots, then let someone else jump in.
janondisttorbrowser.desktop doesn’t work for me either. Isn’t “Use Tor Browser (AnonDist) (default)” one of your choices?
icedove should have been installed by default, then upgraded to thunderbird with a dist-upgrade. Did you install thunderbird separately on your own? Reason I ask is because this 4 year old post says:
Whonix sets those to true by default. Maybe your self-install has different settings although TBH I don’t know why that setting would matter.