Cygnuses are unavailable.

2 and 3. Cygnus overreacts to seeing Shinshoo hurt and possibly dead, she starts crying and the overflow of emotions releases her latent powers. How much emotion have we ever seen Cygnus show? and now with the World Tree residing in Shinshoo's gem, and she gains her power from Shinshoo, it's very likely that she gained more power than usual. The power causes her to transform magical girl style

I want to point something out, here. What happens to World Tree if Shinsoo dies? Afterall, it's in her gem, and it seems to have been damaged in the battle.

Ereve has been attacked before, but her Knights have always managed to fend it off, and then comes someone who is too powerful for them to fight, who kills someone dear to her. It seems clear that her first thoughts would be "We need to get stronger, I need to get stronger"

What I still don't realise is how comes that Ereve, which, according to a quest for Cygnus Knights I remember, is protected by a magical shield which doesn't allow evil individuals related to Black Mage to enter the island, keeps costantly getting successfully attacked and penetrated. Individuals such as Baroq (Master of Disguise of Black Wings), Black Witch, Hilla and now Damien were able to enter the island without any issues.

That was explained with both Baroq (who, maybe, thanks to his appearance-changing abilities, was able to not be detected) and Black Witch (she used Dunamis, a Cygnus Knight, as a means to introduce a curse to Ereve, which likely also allowed her to enter the island easily).

In the Black Witch quest, you also read about that invisible barrier. There, it only says it protect the island from curses, but I'm pretty sure someone (maybe Neinheart) said that it also prevented those individuals from entering the island.

How did Hilla and Damien successfully enter, though? I remember Hilla used her transformation ability (she didn't use her true form in Ereve's conference), so that might have prevented her to be detected by the barrier (even though it wouldn't explain how comes it didn't detect her evil power). But.. what about Damien? I guess we'll have to wait and see.

Hopefully the storyline makes sense, as several things aren't quite clear to me.