What would you call "lizard people" in authentic Chinese?
I think that 蜥蜴人 is acceptable. I found this entry on Baidu which is usually written in Chinese for Chinese people so if that's how it appears there then at least it makes sense to a significant number of people: https://baike.baidu.com/item/%E8%9C%A5%E8%9C%B4%E4%BA%BA/8815302

I would say that a "reptilian shape shifter" isn't the same as a lizard person because there needs to be some distinction between someone who is a 'hyrid' form between a reptile and a human versus someone who can shape shift into that form.