Well to me it sounds like your iPod is....well...dead...the same thing happened to me on my 3G iPod. Your best bet is to just try everything you can think of as many times as you can so that you can get into disk mode. Once in disk mode restore your iPod and the best bet would be to send it to apple. I'm fairly sure they don't like seeing rockbox on a dead iPod.
As for the problem of getting it into disk mode, what i did was just reboot over and over until i was able to get the apple firmware.