Great job on some of these additions, bazzer!
You asked about the steps on the Phalanx, but I see you did a superb job of them already! I would have used fine wire, depending upon the scale, which I would bend with needle nose tweezers and then glue them in place using a small pin hole for locations. It's probably easiest to make the holes before assembly of the parts, and you might even be able to glue the steps in too if you are careful in assembly. Otherwise, I would just make the placement holes before assembly and add the steps after the part is built.
The forward gun came out great, any white edges could be painted as you say. I use both acrylic paints from a bottle and water color pencils to color joints. I use a fine brush with just a dab of water on the bristles on the points of the pencils, and it seems to work okay if you work the water into the pencil color until you get the thickness you want...just like using water colors.
I couldn't find many examples of water tight doors, and the ones I found had to be pulled from photos of larger pics so they are not very clear, but here is what I found. Hope they help, but I rather doubt they will given the blurry images.
Cheers!
Jim