murphy door

I'm repainting/recaulking around our bathroom window and noticed a pretty big gap where a horizontal piece of aluminum trim was cut too short (see pic). I was going to caulk around where the wood meets the aluminum, but the gap (about 5/16" x 1") is really too big to slop in a bunch of caulk if I want it to look nice. Any tricks for dealing with this? I'm not opposed to buying and cutting some new pieces--it just clips into some brackets that hold the window to the opening--but not sure what it's called. Any ideas appreciated. (Ignore the cracks between the plaster and the wood, which will get repaired)

This may be more of a painting issue, but in painting the door casings and trim I noticed that when built the gap between the trim running across the top of the door and the drywall was never caulked. I noticed that the wall paint is flaking on the walls withing a foot of the top of these doors in the one bathroom that was never repainted in 25 years. I had assigned this to the age of the paint in a moist room, but wonder if it is partially due to a failure to seal the top of the door. I am thinking that over time moisture that might condense on the wall could find its way down into onto the outer part of the casing and wick up and out to destroy the paints attachment to drywall. Or is that gap intentionally left open to allow the wall to breathe? Should I caulk those gaps?