I am really loving your build on this armor. My suit was finished back in 2004, and at the time I built it, there was one and only one reference picture. The black and white one from the old 1980s role-playing game Imperial Sourcebook....
I based my build off this. But not having sculpting abilities, I used readily available armor parts. Yes, I skipped the frame around the tanks, the light box on the forehead (instead used the EL-wire lights), a few of the other odds and ends like the recessed areas on each pec of the chestplate that you are doing because I was geeked to get the suit done and rushed through it.
I tried to do some of the proper looks from that picture like the V-neckline on the chest and backplates, and tried to make the shins match. Not having any reference pics of the backside at all, I never knew what it looked like so made it up. This was before they redid that same picture into the colorized version where you can see a second Seatrooper in the background.
The dive knife on my left arm was because I only had one Scout Trooper bicep armor piece so needed something to put on the other side, and in talking to other 501st guys who dive in real life, they said they never go diving without a knife, so that solved my problem. Found a dive knife with a "sci-fi" looking handle, and painted the plastic sheath to match my armor.
My belt was made of little resin boxes in roughly the configuration of the normal Scout Trooper vacformed belts. The separate resin boxes to me made it appear more like a diver's weight belt, which I thought appropriate for this costume.
The reason the suit was a blue shade was because I wasn't personally fond of the black TK/TB armor look at the time and yet I did not want a white Seatrooper because I thought (and still think) these underwater commandos would not be wearing shiny white suits. One of my TK buddies in my Garrison thought a midnight blue would counterbalance the black undersuit well so that's what I did. Midnight Blue spraypaint with acrylic black overwash to make it more murky weathered.
The EL lights on the helmet came about because it looked like the real single reference pics he had some sort of rectangular headlight on the forehead of the helmet, and similar light systems are used by deep sea divers and such in real life, so the blue EL lights were my own version of those diver's lights, but "space style" in the Star Wars realm to give it that "underwater glow effect". I tell you what, canon or not.....EVERYBODY who saw that suit really, really loved those EL lights. They were a huge hit every time I wore that suit !!!!!
Cumberbun like a Scout trooper......my thoughts personally are don't use one, unless it's made of neoprene. Why would Seatroopers need a cloth piece around their midsection like the Scouts who use that section to hold their saddlebags? I don't think they would. Why carry extra cloth around underwater to get waterlogged and weigh you down?
I also would go with a black cheek hose on your suit. It would blend nicely, and be in the same family as the TIE pilot hoses. My own hose I used was a rubber SCBA breather hose off Firefighter equipment. It was extras from a batch the local 501st TIE pilots were using.
Because like I mentioned not having the sculpting skills, my shin armor was built like this.....I started with a base of the old Marco TK shins. These shins didn't have the nice calf sweep of the new TK armors and were more tubular in shape. Perfect for this project. I used Snowtrooper knees because I didn't have any Scout knees. For the recessed side details, I contact cemented pieces of Foamies around the one side of each shin based on the one reference pic to create the depth of layers. To create a more tubular shape around the ankle, I cut a piece of thinner gauge PVC pipe into the proper shape. This is then attached after I'd put the TK shin on. It velcroed under the front center onto the shin. The kneearmor also velcroed to the shin armor. I never got around to making the little jets on the back.
I used a cut down Snowtrooper codpiece and put a second one on the backside to cover my butt somewhat instead of just the open bodysuit since I didn't know what the back was supposed to look like.