Setting ethernet proxy settings without WiFi
This solution worked for me.

Just enter the following command in a terminal (change PROXYIP:PROXYPORT ) 

iptables -t nat -A OUTPUT -o eth0 -p tcp --dport 80 -j DNAT --to PROXYIP:PROXYPORT


Source: https://groups.google.com/forum/#!topic/android-x86/XfIRyHQ5nmk

I had too tried all your commands without success. I was stuck behind a proxy trying to setup Android x86 with a VirtualBox VM which only had eth0. I was stuck at the activation screen not resolving the needed webpages to continue into the OS. After running the above command, I was able to reach the web and continue. Results may vary per proxy environment.

Edit: This didn't get me too far, but it got me somewhere. The same trick didn't work for 443.