Removed those 3 very short sheet metal screws and what I would call the metal chimney came off quite easily. That gave me just enough access to the cavity to be able to vacuum and "toothbrush" out almost all the ash. (Some of it is kind of sticky which makes me think maybe a wasp's nest had been started in there).
Didn't try to remove the tube but that might be my next experiment. Reassembled the chimney and will try it out tonight when it is dark and easier to see what's happening inside that cavity!
Reassured that there is a very heavy metal plate still very sound which separates the tank from the flame. My tank was not being slowly burnt up!
I wouldn't try taking that metal chimney off too often, screws are very short sheet metal screws, easy to mis-thread and not sure what you'd do if one didn't hold upon reassembly.
Will try to attach a few pictures in case somebody else is playing around with a similar issue!