Who said anything about Freud? In any case, you need to ask yourself why it feels good to make an imprint on another human being's mind and soul. There is a reason. It's not random or unexplainable.
I'm not talking about the conscious motives of individual artists. Their conscious motives for creating art may be to "assert [their] existence," as you say.
I would argue that all of our physical and mental traits have arisen from the process of natural and sexual selection. If you accept that, you can also accept that over the generations, we have evolved the ability to be artistic, in many different ways. An individual who has inherited the ability to create good art, and does so, may not (and probably is not) creating art with the conscious goal of using it to attract sexual partners. However, if individuals of the opposite sex do find this ability (or side effects of this ability) attractive, they will be more likely to mate with the artistic individual, perpetuating the artistic genes. This is how I see art coming about because of sexual selection.
No.
This reductionism has no explanatory power. You might as well say "all egoism boils down to the benefit of the other because personal benefit benefits humanity/the universe at large. Therefore all reduces to altruism." You can pick any explanation and say "in the heart this boils down to X". If you have no special reason for choosing X (which is evident if you can explain just as much with Not-X), all such explanations are equally worthless.
There are many options other than believing that a human life divided by infinite space approaches zero.
I have no idea what you mean by a "more aesthetically pleasurable life," except that apparently it's a bad thing, but if we all truly accepted our insignificance (in the face of the cosmos, God, love, whatever) and that we often do "altruistic" things out of self-interest rather than love and compassion, the world would be an amazing place.
There's an universal truth that won't go away soon: we are dying, second by second. We feel good when we "create" something meaningful as it prolongs our existence and it gives us happiness as suddenly we aren't breathing the air for nothing.