1. Your tax questions are best answered by your tax consulatant. On planet earth, different countries have different tax laws. I'm not sure what the milky way laws are but I'm sure a local tax agent could answer them far better than an anonymous pilot on this board.
2. How much do you eat and drink. Tokyo can be expensive, a beer at a bar can cost 8$ or more. If you hide in your room, you can save a lot but the bills for therapy afterward may be high. Count on using up the full accomodation and per diem allowance. Any money saved, will then be a bonus.
3. up to now, the scheduling has been flexible. back to back has been done. The contract specifies tha company can roster 8 days off in a row minimum. This has not been done and they have always done 10 days. The present scheduler is VERY good and tries to accomodate all requests. What the future holds, who knows.
4.This is being revamped by the company but should provide a ticket to closest airport served by any airline near your home. ANA flights will be used as a priority to get you near but direct flights may be given if this makes tie commute too long. New guys in the office applying their interpretations of the rules
5.Generally all crews get on well. The Japanese pilots are very polite, respectful and mostly friendly. The biggest problems have been with non Japanese pilots.
Just be careful. The merger of the two operations could mean many changes, so the future may be very different to past experience. Better or worse, who knows.