Your first three listings of the upper three decks is correct. As for the splinter deck (30 inches below the 2nd deck) is 25.5 lb (5/8" thick) of STS on the Iowa and New Jersey. It was increased to 30.6 lb (3/4" thick) on Missouri and Wisconsin.
Well thanks to Pearl Harbor attack and both Germany and Japan ignoring London Treaty restrictions on standard weights of Battleships, it was open season for how big or heavy we were going to build Battleships from then on (such as the Montana class).
Even the 20mm and 40mm AA machine gun shields were increased from 25.5 lb to 30.6 lb. We needed to increase our lightest armor to at least stop the 7.7mm Arisaka or 8mm Mauser rounds. Hopefully any larger caliber could possibly glance off if not hitting straight on.
The transverse armored bulkhead on frame 50 of Class A armor was increased a couple of more inches on BBs 63 & 64.
Now, the aft armored bulkhead at frame 166 was not the end of the "Armored Box" (as we called it). Where the 2nd deck armor ended but aft of that bulkhead, 3rd deck armor continued to frame 203 to protect the aft steering gear rooms, which also had Class A armor bulkheads port & starboard to protect the sides.
Sorry, I'm a little tired to dig out my armor diagram plans at this time, but just remember: The Iowas were the most heavily armored ships built by ANY nation. Oh yeah, some countries said they had thicker armor. But nowhere did they come close to our Class A armor. Nor did they consider adding the armor several feet INBOARD to use the outer bulkheads and shell plating to act as sacraficial trigger plates or at least slow the weapons down.
As for overhead armor of the magazines, you seem to be referring to the 16"/50 magazines. But all 5"/38 magazines were ON the 3rd deck and only the 6" thick laminated STS/Class B armor was above them. Adding a splinter deck would have taken up too much room and when you have a squadron of suicide planes coming at you -- you need as many rounds as you can get because you have to load those guns with VT/FRAG as fast as possible.
LEAD, DAMN IT! LEAD!
But during their 1980's modernization, I was tasked to design where to add an extra 400 tons of HY-80 armor plate ABOVE the Main Deck to support the Tomahawk Cruise Missile machinery and electronic rooms below the ABLs and the 20mm Magazines under each CIWS "Gatling Gun". That was mostly 30.6 lb armor. Oh yes, also added 40.8 lb armor all around the new communications radio room on main deck about amidships. Then there was the 61.2 lb HY-80 armor all around our new Combat Engagement Center on 02 level.
Oh, almost forgot, also had to add 10.2 lb armor ON TOP of existing 30.6 lb STS armor in a few critical spaces.
Now you may be wondering why I'm describing the thickness of armor in pounds rather than gauged inches. That's the way we have to order it on all of our engineering drawings. That's also the way we have to order it from the steel mills. The poundage shown is what one square foot of steel (of the thickness required) weighs. Comes in very handy for our Scientific Design Section to calculate how much heavier the ship weighs (in long tonnes), where the new center of gravity will be and where the new center of buoyancy will be and how much more draft the ship will have when fully loaded.
Okay, that's my history lesson for today. Still tired out from my birthday yesterday. The crew on the Iowa presented me with a small party and birthday cake and at my favorite VFW post I got more tokens for fee drinks that I could use. (Did use the last one earlier this afternoon).
Thank you for asking some technically critical questions. I'll look up my armor diagrams tor the steering gear room armor tomorrow. I hope.