how are you defining human and people? Usually when people say [b]human[/b] they refer to a certain universalized moral standard, as in someone's humanity. And when people say [b]people[/b] they simply mean a sentient life. If those are your definition, yes a person can lose their sentience and or not abide by whatever moral standard that grants them the title human.
Yes but the definition has "kind of" shifted, a person has a personality, thoughts emotions and can contemplate these, so a person moreso has to be on the level of a human, like how some groups say "animals are people to" they don't mean they are human, just that they are people.
That and in popular fiction where there are non-human sentient species they are called "people" and are regarded as such.