Face unlock missing/camera can't be loaded issues fixed for MI5X, MI6X, HM5Plus, HM6Pro and HMNote5 and ROMs will be reuploaded soon.
About the Android 9 question, we are not Xiaomi employees, we can't answer that.
What do you want us to reply? You did not provide a screenshot of crash summary or a logcat, so we don't know what the problem is.
And my Mi 5 Pro is more powerful than all those you mentioned put together. His answer was correct. Old phones get EOL, and how powerful they are or how well they work for you is completely irrelevant to that decision.
No.
Yes, whenever they decide to actually release what they promised.
It is fixed. However, 120/240fps videos are only designed to work on the stock Mi Video app, so don't be surprised to see that it won't show as slow motion on another player app or your PC.