I figured Red Eye had something to do with The Gate and LOVED it when it showed up and H'aanit freaked out. I didn't work out it was Crossford until the diary though, that detail made it even better. When I fought it, I got the vibe that it looked like a shadow human version of Ghisarma. Maybe he just took the form of whatever beast he fought last since he had been partially absorbed into Galdera and was tripping balls.
Was Darius acting independently of Obsidian? The seal was broken a long time before Heathcoat managed to retrieve the first stone. Therion bringing them all back (and I guess ultimately becoming their new guardian) was necessary to prevent the Gate being unsealed again in the future, but didn't Obsidian know it was already open?
Mathias didn't seem to be clued in about Kit still being an option since he was dead set on snuffing out the sacred flame, but Esmerelda was looking for info in Crossford's diary that seemingly led them to Kit. Can't remember if it said who was funding Mathias, but I think he only ever had contact with Lyblac and she was playing the long game with him, maybe Crossford was an attempted shortcut to her end goal. Using Lianna as a vessel for Galdera outside of the gate, with no Sacred Flame in place, could potentially pose a much bigger threat to the world than sacrificing Kit?