fighting or... he doesn't really do anything... so... age?
mostly due to diseases like white cough or green cough or just battles not mentioned. he is still a young leader.
She died of sickness.
Yes, he died in The Last Hope.
she died of sickness unknown.
She died of a sickness that is never named in the books.
It's rumored to be during a big battle as he saves someone.
Either The Forgotten Warrior or The Last Hope
Not yet. he will lose his final life in the last book of Omen of the Stars.
Nobody knows which book Firestar will lose his last life in, although many think it will happen sometime in the fourth series. And if it will it will probably be in either the first, second, or last book.Erin hunter has said that they have finished making his death scene and she is only on the third book: Night Whisperers.
Nathan Hale
We don't know for sure. She could die in the battle with the Dark Forest or become an elder.
No, when you gain life, your opponent does not lose life in the process.