Good points, which I'll try to answer.

1. I think there is a difference between personal immortality and the continued applicability of ideas. Indeed, ideas often become most applicable after the death of their originator, simply because they attain a degree of separation from that originator, and are thus more "abstract" and "universal." The originator of a particular idea or philosophy is often not cognizant of the full application, simply because he or she is an individual shaped by particular life experiences during his or formative years. This is why (for instance) any debate about whether Mao would personally have approved of contemporary China is irrelevant; SWCC flows from his ideas, and that is what qualifies the modern CPC as following Mao Zedong thought. If the continued applicability of certain ideas beyond the death of their originator is not accepted, we may as well abandon not only Marxism, but any kind of philosophy or ideology whatsoever. It is true that we cannot apply ideas dogmatically, without reference to material conditions; but that itself does not argue against the idea's continued existence and vitality, any more than the fact that one must use a screwdriver correctly argues against the existence of the screwdriver.

2. I tend not to be optimistic about the ability of individual human beings to really adapt to new experiences, and to synthesize them into a coherent worldview -- at least beyond a certain point. In my experience anyway, and a whole lot of history and literature seems to reflect it, most human beings don't really change much after the formative years of childhood and adolescence. Everything after about, oh, 24 or 25 seems to be mostly crystallization, or in the best case scenario, development of what was already there. There are exceptions, but they are rare. Thus if Confucius were alive today, he might not be exactly the same person he was during his (historical) lifetime, but I very much doubt he'd have changed enough to deal coherently with the problems we face. The fact is, humanity changes it outlook as the material circumstances change. Individual human beings often do not.

3. About fear of death being individualistic: we fear death precisely because we identify our essential being entirely with our specific individuality. But this is not, as Marx points out in the Economic and Philosophic Manuscripts of 1844, the truly human way to look at death. Man is a species-being, and our outlook on death (in our particular experience, a human phenomenon), should be from the standpoint of species-being. That is, we identify our true essence with what is common and socially-developed -- i.e., the essentially human characteristics of independence, creativity, and consciousness -- and do not feel any loss at death, because we know these things will continue as humanity continues. This state of mind is not inconceivable; it is that which drives communist martyrs, even those who who were hardline atheists, to suffer death for the sake of the people.

4. The goal of socialism is not specifically to provide a life without suffering, but to provide a life that is, in human terms, meaningful. A level of material abundance in accord with the particular stage of the development of production is certainly necessary for this. But material abundance is a means to an end, for one can be as rich and comfortable as one likes and still be a miserable, useless old bastard. (Just ask Jack Ma, or rather, ask the regulatory officials who had to deal with Jack Ma). Human beings find happiness (in the deepest sense) by serving the people, and a human being's short life is what makes such service meaningful. It is said that "death is the great equalizer," and this is usually meant in the negative sense of "kings must die as well as commoners." But it also has a positive sense, in that it enables everyone to achieve a similar level of revolutionary heroism. Each human being is given their life, and given it once -- which means that spending or giving up that life for the sake of something larger than oneself is a uniquely meaningful sacrifice. This is ultimately, what is meant by a "communist morality:" not something metaphysical, but a recognition of individual meaning in specifically human terms.

5. Your last point is especially well taken, that if immortality is scientifically possible, all this is probably moot. However, as someone who is not a scientist, and doesn't have the leisure or money to become one, I still have to form some kind of tentative opinion on the issue. And it is this; bourgeois scientists have been promising this kind of thing forever, and while life expectancy has certainly increased, individual lifespans have not lengthened all that dramatically. (The Hebrew book of Qoheleth, written between 450 and 180 BC, has the line "the lifespan of a man is seventy winters, or eighty if he is strong.") I find it a bit suspicious that the capitalist media is trumpeting immortality again, right when the system is well and truly collapsing; it seems a distraction like UFOs, or Elon Musk saying that the colonizing Mars is the solution to all earth's problems. I am not totally unsympathetic to an individual's fear of dissolution, but so far, it seems to me that the domain of that kind of thing is still religion and philosophy -- things outside the domain of science, revolutionary science included. There are religious communists, and communists who believe in the immortality of the soul. I respect them as comrades, but am very suspicious of any attempt to drag a metaphysical concept like immortality into the realm of science.