I think Winston is "the last man" because he may have been the last to actually think, judge, deny, desire, and grow ambition. Certainly this book demonstrates that humans are different from animals in our ability not just to be like the proles, but to apply complex thinking. When the ability to think for one's self is completely removed, it seems like a piece of humanity at the very least is removed.
The proles seemed happy to exist, but took part in nothing that caused them to affect change on the world that they took part in. To reach the highest levels of self-actualization in one's life, one wants significance, or at the very least some kind of meaning, legacy or purpose.
I also think humanity means a desire for real truth, whatever that may be. When Winston finally gave in to writing that 2 + 2 = 5, we know his desire for truth had been defeated.
For Winston, to be human meant to experience every aspect of humanity: relationship, faith, purpose, entertainment, and contribution.
Your secondary questions give us a good way to think about your main question. I do not think that we can tell if Winston is the last man because we don't know if there might be others like him in his society, secretly have the same kinds of thought.
But Winston is a man in a way that most of the other people we meet in the book are not. He is human, in my opinion, because he tries to have an individual life. He tries to have his own memories -- to know for himself what things have happened. He tries to have human emotions -- relationships with other people.
I think that these are a couple of things that make us human. It is both the ability to think and the ability to feel that makes us human.