It’s doesn’t have a ton of stuff on it yet but here you go.

I believe it’s Karkat: Contact John

That’s not counting the note that John sent to Karkat before entering the yellow yard.

He became a ghost.

The image for, I believe the 2011 Homestuck calender for the month of March depicted Eridan crossdressing. He is also shown wearing the outfit in the Ministrife flash.  So to answer your question, kinda both.

Not between just the two of them. They both speak in two different memos.  But they might have spoken to one another in one of the walkaround flash games.

I’ve read through it about 2 and a third times.  And I’m planning on reading it again once summer comes around and I have more free time.

Gamzee left it.  We can tell this because he used the blood of a void player (Equius’s) to cover up every mention of himself in the book.

There’s a wiki page for that

We don’t know if it’s impossible for repeat classes and aspects in a session.  But we also don’t know if it’s possible so it might be.  It’s never been touched upon in canon and I don’t know if Hussie ever revealed the answer in his old formspring Q&A’s

Terezi could smell it, and Gamzee saw it.  So either word spread or everyone saw him bleed at some point since they were playing the game for a few weeks, or he just told everyone.