[Audio intro help]

If you tried to run the intro song server side (which idk if is possible) it would require you to stream that song to each client when it is triggered, because of the bandwidth on the upstream channel being consumed it would cause a massive desync problem for all players. The only real option with intro songs is to compress them as best you can before turning them into their required .ogg file type, which is a large file type anyway.

 

Honestly I had intro music in my server and I had gotten it down to 1 meg but it was a short piece, just long enough to last until they were done with the halo jump spawn. Bottom line I removed it since I have allot of scripts that are in the mission file and didn't want to end up bloating users, but every time you add a large file to the mission folder you have to consider is the file size worth the end result.

[Adding custom building items (Player crafted) (need help)]

Well in my experience the best way to incorporate new scripts is to write an entirely new file, and for good reason. If you mess up in the original you will end up breaking all of the modular building stuff in this case on your server. Now to answer your questions.

1. You would literally list the item class name over again in the same requirements field so in this case... 

   require[]={"ItemEtool","ItemToolbox","PartWoodLumber","PartWoodLumber","PartWoodLumber","PartWoodPlywood","PartWoodPlywood"};

2. The function they use to allow player to see and move around with a crafted item before placement I do believe is called ghosting. I have never done it before myself but you can find all the possible sqf commands and what they do HERE!
3. To do it the current way you are doing it you would need to do this inside of your mission file. You would make a copy of the dayz_code\actions\player_build and put this file in your mission then have a custom compiles file that would compile it for your mission with the new correct file path. You would do your edits in the player_build.sqf there are examples breaking down making craftable item classes at the top of the file dayz_code\actions\player_craftItem.
4. According to that file it should be a right click function

Let me know how this turns out for you, I really want to make more craftable items myself but I currently am doing other stuff.

[TRADERS NOT WORKING]

This mainly happens to servers when you create a custom self functions. When there is a syntax error in this file the whole thing doesn't work and it includes trader, unlocking vehicles,....etc. In the interest of not completely breaking your server you should keep a back up copy of the self functions file when you add stuff in and it works, that way the next mod you add in and it breaks something you still are in business.

[How to spawn objects in a certain shape around a player?]

So for one of the scripts I am working on I need to be able to spawn objects around a player in a circle. Think back to when plot poles spawned a circle of tiny pole markers for players to preview the radius of the plot. I wish to do this same function, I just don't know how to spawn items in a shape.