Hawkfrost trained Ivypool in the Dark Forest, manipulating her to turn against her own Clan, ThunderClan. He took advantage of her desire to prove herself and gain recognition, ultimately causing her to betray her Clanmates.
Hawkfrost
Yes He walks there with Tigerstar.
Yes, she does. She dies saving Ivypool from Hawkfrost and two other Dark Forest warriors. While Ivypool is busy fighting off the two Dark Forest warriors, Hawkfrost kills Hollyleaf.
Yes, in Sign of the Moon it specifically said so. That is how Hawkfrost recruited her for the Dark Forest.
No on except the Erins know yet. Sorry... although we all already know that Firestar dies but it is unknown if Hawkfrost kills him.
He claims of not knowing that his warriors participated in Mudclaws rebellion.
1. Tigerstar 2. Brokenstar 3. Clawface 4. Darkstripe 5. Hawkfrost 6. Snowtuft (introduced in Fading Echoes) 7. Shredtail (introduced in Fading Echoes)
No, Hawkfrost didn't go to StarClan. Due to his actions and alliances during his lifetime, he was deemed unworthy of joining the ranks of StarClan after his death. Hawkfrost's manipulative and ambitious nature led him down a darker path, leading to him being denied entry to the peaceful afterlife in StarClan.
Brambleclaw does kill someone. He killed Hawkfrost because hawkfrost tries to take over the forest.
Hawkfrost dies before Firestar, during the events of the final battle in "The Last Hope." Hawfrost is killed by Brambleclaw, who is later revealed to be his half-brother. Firestar dies in the same battle, succumbing to his injuries after defeating the Dark Forest warriors.
She is in the dark forest because she had kits with a cat from another clan. I'm sure that there are some other reasons as well. You can see after reading Crookedstar's Promise that she belongs there though.
I'm guessing Thistleclaw if you read Bluestar's Prophecy, Brokenstar and Tigerstar, of course, and Darkstripe and Hawkfrost, plus more that I don't know of. Maybe Ashfur, but no one knows for sure.