Thanks for these questions Armin, and the chance to explain. I answer in two posts. Here's a summary of the first: 1) I explain the literal necessity of endlessly retelling the myth; 2) I explain how "Life could radiate... but death could not"; 3) I further justify the premise of a supreme value on reason; and 4) I suggest that answers 2 and 3 may help you follow the theoretical deductions.
1. Yes, I mean this literally - "The essential, material practice of rational being is the perpetual telling and retelling of its own, immortal myth" (ab.) - not figuratively. For an interstellar network of civilizations to maintain a slow expansion into the galaxy and beyond, it must constantly remind itself of the reason for this expansion. This requires an endless telling and retelling of its own myth (where we come from and where we are going). Suppose this retelling stops. Then the reason for expansion is lost, and with it the assurance of expansion and hence of survival. You see, barring a fluke of the universe, rational being cannot be maintained in a continuous line without a modicum of interstellar expansion (p. 9); distance and the limit of light speed combine into an effective extinction barrier only when we maintain a sufficient dispersal across that barrier. So the retelling of myth is necessary to rational being.
I'm not sure I understood "stand in". Please repost if you still have a question about that.
2. "Life could radiate across that barrier (just), but death could not." (p. 1) You counter that death too might radiate and "undo" life, but death is a relatively poor self-radiator, self-propagator. Picture an interstellar network of civilizations expanding from the Sun throughout the Orion Spur and beyond. I claim that no mode of extinction could feasibly expand in pursuit and destroy all those civilizations - like a perfect interstellar plague, or a galactic earthquake. While that sort of hazard is conceivable, it's not plausible (see my reply to Tommy, May 1, A).
3. "(P2) Let reason be the supreme value." I justified this premise in the text (p. 2) by its utility in expanding the moral argument to its proper scope. Here I agree with Kant that an ethics co-extensive with humanity would make no sense.
A further justification might hinge on our desire to exist. Nature (P1) allows rational being the choice of existing forever in a continuous line, a choice open to no other form of life. We alone can choose between rational being and non-being. If we choose rational being (and why wouldn't we?), then it makes sense to interpret this choice as a supreme valuation on reason, because reason is the basic difference between rational being and non-being.
I think this premise is strong, though the strength isn't immediately obvious. An artist could show how reason is alive in the mutual gaze of mother and child, and this alone might carry the day (as it carried much of Christianity). A narrow objectivity would see these two as dumb machines, but there's another view, the necessity of which is felt by everyone. Kant's insight helps here. He explains how freedom is subjectively necessary to reason despite its objective impossibility in nature (4:446-63). "Reason must view herself as the authoress of her principles, independently of alien influences, and must consequently, as practical reason, or as the will of a rational being, by herself be viewed as free... From this stem all judgements about actions such that they ought to have been done even if they were not done." (4:448, 55)
So I think reason offers a strong, defensible value-premise in moral theory.
4. Given my answers above (2, 3), it might now be easier to follow the deduction of M0 from the two premises, and thence M1 and M2. The arguments are there, just (as Robert implies, and I admit) much condensed. - Again, thanks for the chance to explain.
(continued in next post)