I kind of skipped through the thread and just read half the posts and looked at the pics, so excuse me if I repeat anything already tried,
But that air intake/vent/baffle.....is it threaded in? Can you remove it? If so, take it off and run the fire with the grate at the same possition as the last pic you posted above. From what I see, with good airflow, as long as your not loosing the heat from around your cooking chamber door, that thing should work.
After removeing the vent cap, if it still doesnt come to temp, place a small fan blowing towards the intake and let us know what that does, if anything.
Id like to see a pic of the fire too.