Latest info comes from the company founder Lin Bin who posted two camera samples on his Weibo profile. They show the portrait mode in action, although they were clearly taken in a very dark environment so it's hard to just image quality based on them.
We also got a teaser video for the Xiaomi Mi Mix 3. A short 15-second clip reveals the all screen front and sliding the rear up will reveal a dual 24 MP selfie camera.
As far as I know, they would start to create a kinda magnetic case for it.
You can see the chin size in the video clearly. It's definitely small, though can't be as slim as patented iPhone's 0 chin.
I'm sure Xiaomi would spoil it by adding the dreaded chin! What's the point of removing the notch if you end up with an even bigger chin?