1. Half and half. Depends on where you commute from and your tolerance for economy class travel. I take the $2000 because I don't have a far commute and usually can get tickets for $600.
2. Only Y class Zeds. Mostly on Star Alliance carriers with a few exceptions. You, spouse and dependents get them.
3. At the moment we are over staffed so short work months are easy to get from time to time. That is subject to change and the only guarantee you will get one is if you work a part time contract. Those typically are not available until after you complete your first contract.
4. Training is not the easiest in the world. What makes it different is the length and the style. It can take as many as 9 or 10 months to check to the line in some cases. Also, you have to remember that this is Japan. If you do things their way and don't argue then it is not so bad. If you have to ask a lot of questions, especially the "why" question, then it will not be a pleasant experience. Yes, there are cases where people do not pass training and lose their jobs. However, if you have patience and humility, then it should be no problem in my opinion.
5. The amount you are in Narita varies a lot from month to month. You are paid an accommodation allowance that will cover the costs of a hotel or you can try to find an apartment. About half stay in the hotels and half stay in the apartments.