@Belisarius II
As far as 18" guns go, the RN proposed a L/45 tube, as opposed to the L/40 wire-wound construction for
Furious. Furious' fired a heavy shell for the calibre at low velocity, much like the 15"; it fired a heavier shell than the ones used by
Yamato's 18.1"/L45 guns. The N3s would have fired a lighter shell at higher velocities. A partial wire-wound and fully built up prototype for the N3s guns were ordered but canceled ahead of completion.
The USN tested an 18"/L48 gun, and was disappointed with the barrel wear, blast effects, slow rate of fire (realistically about a shell a minute), and found that the standard 2900lb projectiles were little better at penetration than a 16" shell, amd thereafter concentrated on superheavy projectiles.
In Japan, sources are conflicted as to whether the gun barrels to be mounted on the No. 13 class would be 45 or 50 calibres long, but all the drawings show them as L/50. None were built or tested.
As for the arrangements, the N3 design was selected because it allowed the shortest possible armoured citadel to protect the magazines and machinery. The turrets were arranged as they were because at the time, it was noted that battleships rarely, if ever, fired their main guns dead astern. If a battleship ever was, something has gone
very wrong. It was thought that most angles could be acheived by firing over-the-shoulder if necessary.
Here is a drawing of the G3 and N3:
and another alt British battleship, the L3 design:
https://duckduckgo.com/?q=m3+class+battleship&t=brave&iar=images&iax=images&ia=images&iai=http://i.imgur.com/3tlfkSd.jpg