As of 'Sunrise', Graystripe is in Thunderclan. If you are asking about where he was when he was taken by the twolegs, he became a kittypet, ran away, and set off on a journey to get back to Thunderclan. You can read about this in the Warriors magna series.
First it was Silverstream but now it is Millie.
It was considered and many cats wanted it, but Brambleclaw remains deputy for now.
Graystripe.
In this book, Graystripe is still prisoner with the Twolegs.
No, Graystripe and Bluestar are not related. Graystripe is a warrior of ThunderClan, and Bluestar is a former leader of ThunderClan. They have different family backgrounds and do not share any blood relation.
Graystripe is not dead yet, when he dies he will join StarClan.
Graystripe becomes warrior.
Of course!! Firestar is Graystripe's best friend.
Yes, graystripe and Darkstripe fight when Graystripe sees Darkstripe feeding Sorrelkit death berries. It's in book 6
No . Firestar's kits are Leafkit/paw/pool, and Squirrelkit/paw/flight. Graystripe is Firestar's best friend and Silverstream was his mate.
Graystripe met Millie at a Twoleg's place when he was captured by Twolegs.
Graystripe has had 2 apprentices, who was Brackenpaw / Brackenfur and Stormpaw/ Stormfur.