Does anyone know how the essay is marked in the Chinese SL exam? I've heard various people say that it is marked on language, structure and content. If this is the case, is it possible to enter the exam with a memorised essay that is perfect in structure and language, but with completely irrelevant content?
This is a serious question! I've devoted no time to the exam and I'm hoping to get a study score in the low to mid twenties.
Chinese SL essays are marked based on the following:
CONTENT: /6
STRUCTURE: /6
LANGUAGE: /8
For a total of 20
Theoretically, you would be able to get 14/20 if you have perfect structure and language, with irrelevant content, but I'm not sure if the assessors will like it if lets say you wrote an essay on 骑自行车的利与弊 for a topic which was asking for 外国留学的利与弊. Usually when you go off topic, it is detrimental to the Language category too, as the control of language and the appropriateness of language may not be spot on for the prompt.
However, if you do manage to get a topic that is slightly similar, lets say “persuading your friend to ride his bicycle to school instead of being driven”, and you had memorised one for "the merits of taking public transport rather than car", then the crossing over of the two topics can net you a few marks. In this case, you would lose primarily content marks. Structure marks pertain to the format/text type/text style and language marks refer to the accuracy, depth and breadth of the language used. You wouldn't lose more than 4-5 marks, as long as there is still some relevance within your essay.
I suggest you to instead of memorising whole essays, memorise body paragraphs instead. That way, you can mix and match them to suit the prompt, and just tweak your body paragraphs

This will certainly get you more marks than completely reproducing an irrelevant essay.
However, that being said, unless you have another 3/4 between now and Chinese, you've still got 7 days left

Work hard until then, and you'll surprise yourself with the result
祝你好运!