About
3) It's normal proxy torp are for Sub/smallship(unbulged BB :D) purpose , they do less damage on BB.
To handle a km:
- make him follow you and shoot rear torp, u need rear torp ( hard in ijn :x) and a good speed;(work good x all T5)(Hook-Line-Sinker)
- make a blind shoot with long range torp, if you guess the right timing the guy will see you when the torp are on him and blow up; getting the right timing and ennemy location are hard. (dont work on High crew player cause they got 40k DP then turn back asap you shooted or pray)

4)well it's happen on other nation too, but look closely all the maxpower have low speed when average got speed and power. I play SN and SS4 is more fit to all combat/pursuit and the SS5 for BB hunt/canT chase.
Cap your engine crew and load low weight. Your Speed intel arent low, some other SS5 like gato run 37 when they are overloaded.

IJN got more sub fit against BB, but that's the nation's choice, or all nation will ask for the same ships.
I guess you must be pretty happy to send 6-8 front torp on BB5 and blow him up but if u had and average Sub you ll need someting like 10-15 torps shoot by 4.

EDIT: Where did i put proxy on IJN? i explained that proxy then logically km equip with proxy was for sub-unbulged ship hunt.
Except km none have proxy but find way sinking by other way except you. You are fit to be Km, just go forward skill play. Skill, brain, ship get use of them i gave you 2 more way to go.
My D-type(SN) is as slow that your IJN in speed and reloading time (dont shoot all in one or aim right) but deadly on BB than my speedy S-type. For your speed did you use an cap-engine-crew? cause when you acquire SS5 engine isnt cap (even with cap VET and premium).
Now they re still your choice in sub purpose BB hunt or multipurpose. Change nation then like i said Gato is in top chart.
(Enjoy how i havent make Learn to play, stuff bignoob and other uncourteous thing)

1)looks fine to me, just predict the movement of your opponent and fire where your opponent is heading

2) cause its a nation's disadvantage

3)it does, it can kill ships easily even with bulge, and km is built for ss vs ss..., km ss waste more torps just to sink a single bb

1) The shell Hang time is (sort of) a advantage in itself because IJN BB4-6 gain more range than others when retreating.

2) I don't know what are you talking about. Structural defense on SY is barely any worse than others (in face it has the same structural defense of a montana).
If you're talking about submergeance then... know that lower tier ships almost always have superior submergeance (take less damage).

3) Dunno don't play subs.

4) Dunno don't play subs

1.) You're forgetting the real advantage of 45 angle guns, a full salvo from max angle from a SY is a death sentence. The hang time is what gives the guns it's lethal punch.

2.) National disadvantage, what comes with speed must be taken from somewhere. Buffing it too much would make it overpowered as a ship.

3.) Again, IJN has some of the most deadly SS salvo's around. IJN SS4 can sink most bb5's with one wave of torpes. Learn to fire your torpes at the range that even if they dud they will still blow up at the ship. IJN is designed for anti-ship combat, KM is designed for Anti-Sub combat.

4.) Pending a full rebalance of the SS lines, I have no comment

So your saying remove all of IJN weaknesses while keeping its strengths