If I hook my e2x0 to a USB power block (so no computer involved) will it still detect USB and start the old firmware to handle charging?
Nope. Rockbox will just keep playing merrily along using power from the power block instead of the battery. Unfortunately it won't recharge the battery. To recharge the battery you have to unplug the power block, boot into the OF and then plug the power block back in. Rumour has it that charging is "close". One reason to stay with an older version of the OF is that some later versions don't allow listening to music while charging.