I've never seen a convoy scatter in the game. It would be cool if they did that on rare occasions. They pretty much stay in their positions but they start to zig zag after an attack or you are sighted for a couple of hours before they settle down again. The zigs can be pretty rapid, maybe even unrealistically, and you have to be pretty careful shooting while they are zigging.
The only time I've seen the formation get pretty confused is when the convoy hits a waypoint and changes course. I was moving in ahead of a convoy for an attack once when that happened and I had to get out and wait till things settled down before moving in again.
I don't know for sure, but you may be able to keep your current career when you DL GWX.
Also get SH3Commander. It is a game aid that increases immersion and adds some nice settings for gameplay and career options.