StarClan told Midnight about the danger looming over the Clans because they believed that the Clans needed to know in order to protect themselves and survive. Midnight served as a messenger because she had a connection to both the natural world and the spiritual one, making her a suitable conduit for StarClan's message.
Yes, they were from Starclan. They were trying to decide who will journy to the Sundrowned place.
No he does not, because he knows they are kin. Starclan told him but he was told not to tell anyone
Mosskit did join Starclan as you will learn (if you haven't already) in Warriors Field Guide: Secrets of the clans. He died while Bluestar was bringing Mosskit, Stonekit, and Mistykit to their father: Oakheart of riverclan.
Midnight is a wise badger that speaks cat. She knows a lot about StarClan and warned the cats about the Twolegs destroying the forest. She also warned Leafpool about the badger attack from her kin. That's what made Leafpool and Crowfeather go back. Then Midnight told WindClan to go help ThunderClan.
She went to starclan since in the lost warrior she is seen in starclan with Silverstream.
She's dead, but it is not know whether she is in StarClan. She probably is in StarClan. She didn't do anything that's Dark-Forest-bad.
Yes, when Honeyfern died she went to Starclan.
Firestar escorted Bluestar to StarClan when she passed away. He was a close friend and former deputy of Bluestar, and he guided her to join the ranks of the ancestors in StarClan.
If the cats are good even if they don't believe in StarClan they go to StarClan, but the cats like the cats from BloodClan don't. I'm not sure about the cats that have never heard of StarClan, but are good. Even if some cats believe in StarClan, but they do some really bad things, then they go to the "Place of No Stars".
Graystripe is not dead yet, when he dies he will join StarClan.
I think they did, because they sent spottedleaf a sign saying- (fire alone will save our clan) the fire was going to be fireheart, who saved bluestar and became the deputy. starclan new Starclan can't see everything. They probley didn't know. yes,they knew. fire alone wiil save our clan.
Blackstar eventually does turn back to believing in StarClan. After facing hardships and questioning his beliefs, he embraces the notion of StarClan and the warrior code once again.