I will try to answer your questions.
1) You can choose in whatever you want to spend your dragon kill points, if you want to put 10 points in forest fights you are free to do it, you win 1 point each time you kill him.
2) Yes, you win some donation points each time you defeat him, is not just for the first time.
3) You can choose the kekkei genkai you want, this is something you must work hard to get, so just killing him one time is not enough.
4) You keep your charm each time you beat the boss.
I hope, I helped you in something, anyway, you should check out the wiki, it is very helpful when you have this kind of questions.
Here is the link:
http://wiki.shinobilegends.com/index.php/Main_Page