An OC at stock voltage is fine! If you can lower your voltage, you can raise your core clock instead!
Yes, lowering the RAM speed does decrease temps a bit!
Lowering the voltage won't damage anything! Though it may introduce instability!
Same VRM but multiple output. This is always the case because RAM & GPU operate at different voltage.
As others have indicated I have found no way to lower voltage (other than an unsupported bios flash). Even utilities like ATI Overvlot and XFX Overvolting tool which allow undervolting core only have options to RAISE memory voltage.