Welcome to Subsim!
1. Japanese hydrophone operators are OK. There will be a range of skills you will encounter all the way between Superman and a rock. What the game deals you with is what the game deals for you. I'd run silent.
2. Your torpedoes can only turn 135º. That means that if the stern tubes bear 180º from the target, the torpedoes can't get there. The AI should refuse to fire. You'll have to rotate the boat around. That will make confirming your setup data, especially target speed important.
As long as you have a half decent range in the TDC you can do a Dick O'Kane with your sub running parallel with the convoy. This is a good news/bad news situation. The good news is that you can fire 10 torpedoes. The bad news is that broadside to the escorts you might have very little time to shoot and the targets may be alerted. In a convoy situation you can just fill the water with 10 individually targeted torpedoes and guarantee a decent amount of hits on something though.
3. No, I would aim at the big ones in the center from outside. If a little one gets in the way then it's God's will. Heh, heh, heh!
4. Heavy cruisers and seaplane tenders are of first importance. Large merchies next. In the end, you take what the situation hands you.
5. About 300 yards. Different mods tweak this a little. 400 yards has been safe in any configuration I've used.
6. Never overreact. That is just as deadly as underreaction. You only go full speed with violent maneuvering when you are reacting to an actively hunting close-aboard escort. Until then, keep your cool, stay at periscope depth and keep your combat awareness. A valid situational awareness is the most valuable defense. As soon as you go deep you lose this. Always do things only when necessary. Never assume you've been spotted. Never assume you haven't been spotted. React to the evidence.
7. Until you die. I have almost never taken any boat below design depth. When I did it was never below 50' below design depth. And that's with TMO. With RFB, it should be even less necessary. Be dangerous whenever possible, I say! Hiding subs aren't dangerous. When you are hiding, always be asking the question, "Why shouldn't I return to periscope depth RIGHT NOW and regain the initiative." The deeper you are, the longer it takes to do that.