What about merging Utapau with Mustafar?
Obi-Wan is officially there to bring down Grievous who falls to an uncertain end in a lava pit, Anakin is unofficially there to kill the Separatist leaders (thinking Obi-Wan) already dead.
That way you could blur the identity of Vader by insinuating it might be Grievous reborn and still have Anakin (not yet Vader ) fight Obi-Wan because he is a threat to the Chancellor and have a suited Vader enter the Temple and dubbed Lord Vader.