I know Hussie collaborated with artists but did he write everything himself? Also what was the last art he did for the comic?

The stuff leading up to Terezi: Remem8er was his last art contribution I believe, although he was still involved in the making of the other panels and flashes and such and I believe he animated part of Collide as well?

As far as I know though, all of the writing in Homestuck was by him.