Well Henry I can answer number 1
1: The damage model used in the GW mod is very simple
Stock SH3 uses Hitpoint-damage model so a lot of simple shots and eye catching Explosions.
the GW damage model works with Floodings like in ur sub. if you shoot an compartment of an ship it creates floodings. so if you shoot on different spots of an ship it will create floodings causing the ship to be pulled down to the bottom in place of blowing up. but also this damage model requires some patience, sometimes an ship can take hours before she decides to go to the bottom. so shooting any more torpedo's at an ship could be an waste.
but still the ships have their weak spots (Fuel, Engine, Cargo stash Ammo/Explosives)
also about number 2, in GW I havent experienced carzy pinging like what ever I do they keep pinging me. also you must know that you have to keep the DD 0º or 180º (bearing) so the sonar doesnt have a lot of Shape of your sub to catch.
Also when you have been detected and get DC's play death, Silent rig dive deeper if posible going 1knt.
in the Stock SH3 the DDs' have gods ears. they can hear an crewmember fart in ur sub it doesnt really matter what you do they find you anyway. in GW this has been made a little more realistic so you still have an chance to get away with ur Guts inside you. but still the key of survival lies in ur hands
good luck