MBP early '11 kernel_task over 300% when unplugged
Something is telling the kernel to do work.

You might need to run sysdiagnose and get expert assistance to interpret the results. Since you've done so much troubleshooting, it's going to be unlikely someone can drift in and point out an obvious flaw in your reasoning (but if they do - we might have a quick resolution.)

The real data missing is what you've done to isolate the issue. Can you summon this on demand? Does it happen immediately after a reboot?

You'll need to have some process to keep track of when the issue arises - once you have 5 to 10 events, then it's much easier to go over the system.log and top process listing to see if any patterns emerge.

A single sysdiagnose is very hard to pinpoint a problem by a non-expert. Ten events where sysdiagnose was run and there is data on what was connected, what you were doing, and ten sets of detailed logs is often very easy to narrow down your specific issue.