Hi, welcome.
1: Any chance you challenged your crushdepth? You'll take damage when you cross a certain depth. Which subsequently reduces your safe depth. So your hull can deteriorate quickly unless you go up quickly.
2: Yeah, don't relly on your sonar operator that much. You with your own ears can hear contacts from much further than they will report. (No offense. But I'll keep a small caveat with some one that calls himself "Grandpa".
)
3: Contacts can be loners, or part of a convoy. The loner, or the entire convoy will follow a scripted path. Though there may be some forks along the journey to a secondary waypoint. Which is chosen depends on the random number god. Asside from that, there are indeed certain routes between landmasses that ships follow. Concentrate on chokepoints to deliver your fish. Keep an eye on markers that appear on the map at random times.
If you find several ships close together moving in the same direction then it is likely a convoy. You can have multiple loner ships in your vicinity but that is not all that common.