Gristmas is fanmade, the 12th Perigee’s Eve is canon.

I’d argue that he doesn’t know how quest beds work and that he put Jade on her quest bed because whenever Jade would have a narcoleptic episode Bec would always put her in bed.

The Cali’s were raised on the meteor on Earth in the distant future so Gamzee just had to fast forward along the meteor’s timeline to reach them.

He got them by touching a very powerful juju

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.