Addressing problem 1. How to "flesh-out" protagonists that don't live in the flesh?
Approach 1. Maybe my protagonists could live in virtual reality. That is, while they are crossing the unfathomable depths of space incarnated as rays of elementary particles, they could imagine that they are something else somewhere else, floating in some kind of consensus reality, projected by the same computational devices that encode their personalities. That way they could build all kinds of imaginary worlds and imaginary bodies for themselves; without human constraints, their interactions could be even richer than they are in the human world.
Advantages of this approach
Indeed, if one lives as a ray of particles, which sounds like a rather boring life, wouldn't they want to spruce up their bland reality with a little imagination, by pretending to be, say, a giant squid or a cartoon character?
Something in me whispers, no, they wouldn't. Their reality only looks stark to us, meat-humans; the encoded personalities would probably see it as something entirely different and much richer. To paraphrase a Confucian saying, to suggest to them that they may add variety to their lives by re-enacting a fictional reality based on human imagination would be like asking a lion to become a cockroach. :-)
But this approach would solve all 3 of my problems (which I talked about in the two previous posts) pretty well. It's obvious how it would solve problem 1: in a virtual world the characters can be fleshed out in as much detail as I want. It would also go a long way toward solving problem 2. Whatever idea you come up with, it's easier to show its consequences in action in a virtual world, governed just by your imagination, than in a physical world with its "hard" constraints. And problem 3 can be avoided too, because a world governed by your imagination does not have to match the reality your readers know. So you can get away with knowing fairly little about it.
And since I write my stories for my contemporary human readers, not for the actual future humans who may encode their personalities onto rays of light, I need not try to attain a realistic portrayal of the existence of photon-encoded personalities, the way it may actually be some day. (As if I could). I only need to make the story entertaining, and the characters convincing, to my contemporary humans. And if lions have to become cockroaches in order to become accessible to us, so be it.
All in all it seems not too bad an idea, to let my "abstract" protagonists live in a virtual reality of their own creation. And they may even have a reason to want it, given that they originated as humans. Their personalities were copied from actual human personalities. So they may still have some human sensibilities (until they discover how much richer is the reality once you eliminate human constraints).
This approach is basically a one-trick pony. I can set one of my stories in virtual reality, but not all of them. I already tried this device in one of the stories I worked on this year. After spending several months on it and rewriting it in at least two different versions, I was so unsatisfied with it that I pushed it way, way to the back of the back burner. :-) Also, it ballooned way beyond a short story and into the territory of novella. In which case, I thought, hell, why not make it a full-blown novel? But it will have to wait until I have built stronger writing muscles. Because in the state it is now, I still haven't found that proverbial Archimedes lever for this story that would move its world. :-) (Even though I've mulled over the ideas suggested by my friend D, I still can't satisfactory wrap my mind around it. One of my fonder wishes for the New Year is to find the Archimedes lever for this story in time for the next year's NaNoWriMo. :-))
Regardless, there is a limited mileage I can get out of the virtual reality setting. After I used it once, I won't be able to reuse it. So I need to think of other approaches as well. More about them in the next post.