Thanks for sharing your story :)

Grml ships the resolvconf package out-of-the-box, so you should be able to set
"dns-nameservers 8.8.8.8 8.8.4.4" in your /etc/network/interfaces file and that
should play well together with the "nonetworking" boot option. Can you please
verify that this solves your issue?

I'm confused why grub2 works in your situation though. Can you please verify
that the same boot options are present with grub2 boot vs. isolinux boot in
/proc/cmdline?

regards,
-mika-