I am afraid, this will be a long response.
It's a lot to respond to indeed, and I wish I had more time to do all of it justice... I believe I've responded to a lot of this in my response on my posts, but I will make an addendum here.
Though I would take a different term, but when I make my own definition of "communism" it means only to be embedded in a community I can grasp locally and physically, so I can feel I have an influence and can be seen and heard, so I can see and hear the rest of the community.
Here you strike the essential point where we've seen communism fail and succeed throughout history. It only ever succeeded in the small communities you describe here, like it did in prehistorical tribes, which were communist if we were to give them a label. And I don't know how to expand the community of human beings to the scales needed for levels of production that would make "modern communism," for lack of better words, possible. All institutions we've known so far have always become corrupt. Not even always intentionally, but simply because at some point the institution does thing for its own survival and its own power, rather than for the people it was intended for. Frank Herbert, the writer of the Dune book series once said something like this: "It's not that power corrupts, it's that power attracts the corruptible." This is the tension between my dream of communism and all the ways we've devised to organize large groups of people into a functional whole; such organization has never worked without the use of power and hierarchies. Take the nation; a nation is built around fabricated unity and loyalties, and fits within fabricated lines on a map that's not the real place, but merely a representation of it. Flags, national anthems, national sports, they're all thought up by those in power to foment cohesion between people who sometimes, or even most of the time have very little in common.
Still I believe it's possible. Call me a dreamer. But I do believe it's possible to teach ourselves to see in each other the ways in which we're the same (which is a lot), instead of always hammering on how we're different from each other. Our individual uniqueness is at the same time something that makes us all the same: we're all equally unique, are we not?
Your point about science is well taken and understood. However, I blame the fact that scientists stray away from their true calling and compete with each other to, as you say, "make a difference in the world," on the turbo-boosted individualism ingrained in us by more than two centuries of capitalism. And I know we can do better than that...