Thank you.
Can confirm that installing the official (Mabox’s) gtk-nocsd-git fixes the LD_PRELOAD message when generating a new ISO.
The line in ~/.bash_profile is present in the new ISO only, not in the VM.
The only remaining issue is the Internet not working (WiFi or Ethernet) even when it is connected, gets the IP and ping successfully to public DNS (ie. 8.8.8.8). The temporary fix, in the meantime, is resetting NetworkManager. Still couldn’t figure out a way for penguin-eggs not doing this unwanted behavior on new ISOs.