Welcome to Subsim, Noxxy
1)
Congrats for sinking one, they are difficult targets
For their historical accuracy in the game, I don't know, I will let others jump in.
The reason for no entry in the log is that the target may have sunken because of bad weather (despite you hit it with your torpedos). Was there rough sea? In this case you won't get credited for sinking, also applies for other enemy ships, that's a shortcomming of the game, but it should happen only rarely.
2) In the game options you can toggle stabilization for optics. Switching it on will reduce your Realism %.
3)
What do you mean exactly? External camera can be used as a cheat but doesn't have to. It's mostly there to enjoy outside views in non-combat situations wether you play realistic or not. Using it in combat is pure cheating though. That said, I must admit that I sometimes use it to identify the ship flag, yeah shame on me...
4)
No for the mast, but you can manually set the range in the WO window, or you can use the book, use the icon on its lower right (difficult to see) which will send data to stadimeter, then you should be on the stadimeter to set the range in the WO window.
(But I don't know if it works like that when playing at 100% and in real navigation)