It is covered with foil, and there is a hole in the foil so the grease can drain out, although I usually put a pan under the big piece of meat to catch the juices, so the pan hasn't gotten very dirty.
I don't know how much the air from that hole contributes to the air flow through the smoker, compared to the three holes in the chip loader. Interesting question.
I just finished cleaning the inside and while it doesn't look a whole lot different, I think I got the worst of the top layer off, especially on the door which had several times the build-up and which seemed to have stuff that didn't smell too nice. Also, it came off almost completely, whereas the black "varnish" on the interior was quite thin, and didn't want to come off.
And speaking of smell, I always leave the door open for several hours after a smoke to let it dry out from the moisture coming from the meat, and then I heat it up for about 30 minutes, and then let it dry out again. Even in a relatively dry climate like ours (central CA), this "dorm room refrigerator" smoker cabinet is going to mildew just like it would if it WAS a refrigerator and you kept the door closed for several weeks and left it at room temperature.
As a result, don't have any mildew.