In my experience working with students to prepare for English-level exams such as TOEFL or IELTS, the testing examiners are not just looking for big words or complex sentences. Yes, they will look to see if you use only basic sentences and your choice of words. They are more interested in how well you express your thoughts and ideas and how you support your ideas throughout the writing. Advanced vocabulary is only good if it is used properly and in the correct context. I would recommend that you focus on writing using words and sentences that you feel are appropriate, not trying to guess what they are wanting and maybe not showing your true abilities.

Your writing here is a good balance! Like you say, you don’t want to write lots of long sentences together. It makes an essay difficult to read. Use less common vocabulary, but make sure you use it accurately.

I agree with all the other answers. You wrote "the exam". My question would be "Which exam?" I imagine that your teacher was suggesting that you do SOME of those things.

I'm confused. Is authenticity defined as complexity? And is complexity thus the exam criteria to meet? If so, I'd focus on meeting those criteria and forgetting about real world authenticity, since that's hardly how things really work. You're also wise to the issue of making more mistakes when trying to artificially make something complex for the sake of complexity itself, so well done. Academically, things shouldn't be needlessly complex, by sacrificing clarity. But perhaps that's a question of different standards between countries. So if it's just about making things complex, then simply write very long sentences, add in lots of exceptions to the examples you put forth, avoid simple and common words and make indirect references to what you've said: The Chinese state, henceforth known merely by its ancient name China, is itself an amalgamation of states subsumed by former warring parties whom within the bounds of the territory formed bonds of inexplicable depth and kinship ties lasting for what may to the observer seem akin to eons, in order to correctly promulgate an extensive, not to mention surreptitious, implementation of statecraft rarely noticed due to its degree of imperceptible attention to detail in its manner of orchestration along essentialist organisational lines as previously alluded to, regarding kinship. Do I pass? 🥴🥲