Oh God. See I'm a big gentle parenting advocate generally, but this seems to me the worst of both worlds:
(a) talking to her like she's a toddler, not a 9 year old (maybe it's all the exclamation marks and 'oh no!', but I'm hearing this in such a babying voice and that isn't appropriate for a 9yo).
(b) putting all the responsibility on her to figure out what she has to say/do to get her treat back - it's like a test or a trick, given that obviously not whatever she suggests will cut the mustard in the OP's book nor should it - if the DD says "Well why don't we forget it and you just fill my calendar like normal", which is a pretty obvious solution from a child's perspective, she's not going to say yes, is she? So the child has to keep 'guessing' until she guesses right basically. Feels very manipulative.
The girl needs clarity, and consequences, and an appropriate way to make amends. She needs this scaffolded for her, because she's a child and thus pretty low on introspection and empathy, but she doesn't need to be patronised like a toddler.
OP, the obvious advice is not to make a threat you're not happy to follow through, but I do this all the time so no judgment here :P
Now you are where you are, I would just be straight with her.
"I felt very angry and upset when you refused to help the family by tidying your mess, and when you were rude and unkind to me and to daddy. When we're angry and upset, sometimes we say things we don't mean - you said some nasty things to me that I'm sure you didn't mean; and I told you I wouldn't give you your advent calendar. Now I've calmed down a bit, I've realised that isn't how I want to deal with this - I don't want us to be fighting all the way until Christmas, and I don't want to make you sad every morning when DD5 opens hers and you don't have one. That isn't going to make me feel better about what you said and did, and I don't think it will help you to do better next time."
Then have a conversation with her about reasonable expectations within the family - talk about the things you and her father do to make life good for you all, and explain that you need everybody's help to keep it that way. If she doesn't already have regular chores, then give her some - so if tidying was just an ad hoc "It's a pigsty in here, tidy up before dinnertime", instead turn it into "you and your sister's job is to make sure the floor is clear of toys etc by dinnertime every day. If this isn't done you lose X privilege" - and this should be something short term and regular, rather than a punishment that will run and run for ages and torture you all. Something like "you can't watch TV before bedtime" or even the dreaded "no afters" - whatever has value for her but won't drag out forever.
As and when the system fails and she doesn't do her chore, and throws a fit when you tell her to do it, you just remind her "This behaviour is not acceptable, it's not how we talk to each other in this family. It's ok to be angry, it's not ok to call me names/threaten me/say unkind things to me etc." Don't get involved, don't argue, and don't 'punish' her for what she's said, just hold the line on the task and the consequences if she doesn't do the task, and don't let her see the unkind talk is getting to you. She's doing it to get her way, to either to be let off the task or let off the punishment. So when neither of those things happen, and all she's got for her trouble is a sore throat and a mother who, whilst not getting all het up, doesn't want to be around her when she's behaving like that, she'll start learning the only one suffering for it is herself. The good thing about this as well is, if you already have a system with a set expectation and a set punishment, rather than a more woolly expectation that she 'be nice', it avoids you having to come up with something nuclear on the spot that you then don't want to have to hold yourself to later.