Well there are some nice things about the mission and some that need work.
1) While I appreciate the effort involved in getting the freighters to cue up like that it's a little unworkable. Firstly they are too close together. With FS2_Open the flames from one freighter are actually in the container of the one behind it. A more standard or pattern would look better and would be easier to protect (and therefore a more sensible formation for the convoy to form.
2) No messages are transmitted on losing a freighter making it hard to gauge how you're doing. Each freighter should have a message if it's lost (and also one when it starts getting to around 30% for that matter). Unique messages are good if you can do it but avoid repeatition of any long distress calls.
3) Consider setting all the freighters to a lower escort priority and using the damaged-escort-priority-all SEXP so that the player can see which ships need protecting the most (rather than simply seeing the ones you've assigned the highest priority to)
4) While you've done a good job randomising their arrival time and entry vector t's fairly obvious that you've only used one enemy wing with a lot of waves. Rather than having ten waves of Leo it might be nicer to have 3 waves and then have a seperate wing come in so you don't end up facing Leo 38.
5) There doesn't appear to be any way to lose the mission apart from being killed. Make it so that a certain number of freighters must survive. Have debriefing that complains at the player for losing valuable ships and cargo if they fail
6) Having to jump at the nav bouy is new. Make sure the player is aware of it and also why he must do it (The company only paying for inferior jump drives will do). In all other missions the player has been able to jump out wherever he liked, facing in whichever direction he liked. If you're going to change that you need to inform the player why and what he has to do a little better.
Apart from that there are some things about the mission I liked
Keep it up.