1. There is the possibility to change the start items that you get the first time you login to the server, this feature is going to get added to the NetherServerLauncher next time we push a update on this.
2. No, 0 will disable it, what you can do is put it on 1 dollar, which is almost free.
4. Most of them are for internal use, just keeping a record of what "versions" or other counters, nothing you can really use, but there is one:
"GiftItems": , //These are the starting items
And no, there is no public reference at the moment on items, but hopefully we can get the next ServerLauncher in the next nether update, if so, you will have a list of items with their names you can add to the starting list, and change the store price, etc