You see crawling one multiple times in BS 1 actually. After defeating nameless demon and summoning dorothy, at the end of D end, after D end and talk to jabberwock. Prickett is an avatar of crawling one and was the Alice in the tea garden party which happened before BS 1.
C end is the end of the story Mary Sue wrote for Grimm to play the lead role in, so essentially it didn't matter at all. If you watch Leaf's character ending after getting C end, it gets extended. D end happens though and crawling one puts a stop to it all which leads into BS 2. BS 2 takes place some time after D end.
Mary Sue thought she was safe but then Grimm invited Baphomet to the holy garden which caused a lot of chaos and let crawling one do its thing.
1) I feel like endings in BS 1-2 are meant to be achieved in order from A-D and A-H. It would resemble progression, changes of Grimm's/Carroll's mind and power. For example in ending A Grimm tried to care the most about everyone, saved everyone and was praised with a party. Then he went into second loop and achieved every female ending, then got bored and cared less about everyone, gaining ending B. Then his mental state/boringness worsened and he even tried to kill everyone, gaining ending C. And after he got powerful enough, he went into Black Trial (progressively the most difficult location to get to and Baphomet tells you to kill Prayer Master, the strongest character in the game).
Thus I think maybe returning fairy tales changed something but Crawling One decided to attack Mary's garden when it was in the most miserable state to risk less?
Mary Sue stole tales from The Crawling One, meaning she stole his power as well. After you return tales, you made TCO return it's power. And destructing Garden was needed to gain control over it and remade it, what Crawling One actually did I suppose.
Yet technically you can reach any ending anytime, I know.
2) Why do you think that Prickett is Crawling One's avatar? For me it feels like Alice or half-Alice is being on a leash of the Crawling One, doing all dirty work, while Crawling One gets all the spoils. Prickett even tried to kill herself many times because such position sucks. Mabel referred to her as a toy. Or am I deceived?