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.
Yes, yes she does, if you're talking about the Warriors Bluestar. Her kits are: Mosskit(now in StarClan), Stonefur (Starclan), and Mistystar (Riverclan Leader).
No, Mosskit does not become an apprentice. He remains a kit. When Mosskit dies that cold night in the forest, no cat in Starclan wants to take him because he's half-clan. Bluestar's sister, Snowfur, decides to help him. For more details see Secrets of the Clans, Snowfur speaks: A Sad, Cold Death.
Graystripe is not dead yet, when he dies he will join StarClan.
Oakheart picks them up at RiverClan and raises them to be Stonefur and Mistyfoot.But one of their littermates, the runt, Mosskit, dies on the way because he is weak and Bluestar's sister, Snowfur, in StarClan takes care of Mosskit.Mistyfoot and Stonefur went to RiverClan with their father, Oakheart and adopted mother, Graypool while Mosskit died, Sad! ~Tinystar~
oakheart came and brought her to starclan.
While we don't know for sure whether Hollyleaf joins Starclan or not, there are three main theories on this topic. The first theory says that she does not join Starclan because of her actions just before she died. The second theory says that she will still join Starclan in spite of this. Another theory says that she is not dead at all, but is trapped beneath the tunnels.
The Darkest Path.
It is not confirmed when, but it will happen in Omen of the Stars
In the warriors series, the cats that join Starclan are those who die and have followed the warrior code, and served their clan faithfully.
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".
Mate:Thistleclaw:[2] Deceased, Residence UnknownSon:Whitestorm[3]: Deceased, Verified StarClan memberFather:Stormtail[4]: Deceased, Verified StarClan MemberMother:Moonflower[5]: Deceased, Verified StarClan memberSister:Bluestar[6]: Deceased, Verified StarClan memberUncle:Goosefeather[7]: Deceased, Verified StarClan memberNieces:Mistystar[8]: Living (As of Fading Echoes)Mosskit[9]: Deceased, Verified StarClan memberNephew:Stonefur[10]: Deceased, Verified StarClan memberGrand-Nephews:Reedwhisker[11]: Living (As of Fading Echoes)Gransons:Sootfur[12]: Deceased, Suspected StarClan memberRainwhisker[13]: Deceased, Suspected StarClan memberGranddaughters:Sorreltail[14]: Living (As of Fading Echoes)Great-Grandsons:Molepaw[15]: Deceased, Suspected StarClan memberGreat-Granddaughters:Poppyfrost[16]: Living (As of Fading Echoes)Honeyfern[17]: Deceased, Verified StarClan memberCinderheart[18]: Living (As of Fading Echoes)Great-Great-Grandkits:Molekit[19]: Living (As of Fading Echoes)Cherrykit[20]: Living (As of Fading Echoes)
Yes, when Bone, who was killed by many apprentices, from BloodClan, killed him.