Those are all very different questions.

What makes a human? Well homo sapiens, the primate species. if you have the requisite genetic structure, DNA and chromosomes etc... Then you're a human.

A cloned neanderthal would not be a human... It would be a neanderthal.

You mention humanity... Now that is far more subjective, and more to do with the difference between how you socially treat a human vs an animal. Hence if you were treating someone inhumanely, you would be treating them in a manner equivalent to (or less than) an animal. This of course exists because most humans elevate their importance above all other animal and plant life.

Sentience is a very tricky thing to define, and is normally defined as awareness and the ability to experience feelings or sensations.

Of course this definition came about back in the 1600s, feelings and sensations are just a bunch of electrical impulses interpreted by your brain in order to try and get the human to act in a specific manner because at some point historically, acting in said manner would have increased the odds of survival. So does it apply to an android? Depends whether you insist on keeping the words "feeling" and "sensations" in the definition...

I personally wouldnt justify keeping a gorilla as a slave (assuming it wouldnt just tear my arms off) so I certainly wouldnt justify neanderthal slaves.

This can be applied to the modern day. Do you think that the great apes deserve "right to life". If so, what other animals would you extend that to?