Regarding question 1:
I have no idea why it is that complicated, but feel free to experiment with other combinations ;). You du not have to add your new package to all the stations. Adding it to these you want it buyable on is enought.

Question 2:
To make a gun a turret, the only really important thing is on what kind of hardpoint it mounts. You can change the model, as all vanilla turrets strangely use the same one, but this is definatly not necessary!
A different name usually comes with it, too ;).
Another difference between vanilla turrents and their weapon aquivalent usually is the higher projectile speed, probably to make it easier to shoot at targets behind you.
LOD means Level Of Detail, the numbers in there define the distances between camera and model between which differently detailed 2d models are used.
I guess in your case the easiest way would be to give your freighter the ability to mount class 10 weapons on its turret hardpoints?

Question 3:
In Freelancer\EXE you can find several resource dll files which contain the names and descriptions. There are different programs to edit them, which work in different ways or offer different features - have a look for fled-ids, or flDev for example.
So if you just want to add a few things, you should be able to find free spaces inside the existing files, but you can also start a new file - there’s an empty one available somewhere here, I think. The texts themselves are adressed by these numbers you have already seen - I think the best way would be to have a look at existing ones to see how formatting works. Sorry that I can’t give you more in-depth help regarding this topic, but it’s been a longer while since I last activly had to do with this game, and I don’t have the files on this computer to test myself.
Have a look into the different tutorials around, there may not be one exactly for this topic, but it may be covered in system or ship creation. It has to be somewhere, as I learned it via tutorials myself 😉