I had thought of questions like these myself, me1.
When no answers are forthcoming from anywhere then you keep the questions there, on hold, as it were, reflecting on them, even if only passively. And it is not that the ‘answers’ are right first time around. The initial ones may only serve to ‘set the ground’ and to dispel any wrong implications taken for granted in the questions themselves.
This is what I have ,and that only up to now because I stand to be corrected, either by others or by the facts themselves when these come to light. As Lal here and others never tire of repeating: it will be most beneficial, vital, indispensable that you concentrate your efforts where it truly matters – for your own future well-being and that of others, and that is, in striving to eliminate greed, ill-will and delusion. All answers that we get to these questions, be they correct or incorrect, will not help one bit in the quest of Ariya-hood, which is the greatest, and hardest, quest one can possibly pursue. And the time for that is now, during a Buddha sasana, during which it is extremely rare to find oneself in a human body. With that in mind, here are my views:
There in nothing that is fortuitous or to no purpose in Existence. “Theoretically all planet systems could be inhabited”…granted, but we need not bring in extra-terrestrial involvement here at all, although I am certain myself that happens all the time, and that even Earth has been visited any number of times, and by more than one type of extra-terrestrial humans. And I am talking about visits in spacecraft and physical bodies. Consider: what would the Earth look like in its (re-)formation phase(s)? A ball of bubbling rock surrounded by impenetrable clouds of dust and gas most probably – making it a scientifically typical model of ‘ a planet unable to evolve life and sustain it’.
“Theoretically all planet systems could be inhabited”.. yes, only omit ‘Theoretically’. Even those planets which science deems unable to sustain life because they orbit their parent stars outside the ‘Goldilocks Zone’ – they will harbour life later as they must have in the past. Life will adapt to ANY prevailing conditions. We have proof enough of that here on this planet. We see lifeforms that thrive in Saharan temperatures, others in polar conditions, yet others in the depths of the oceans under tremendous pressure and in total darkness, and still others without the need of air(whether derived from the atmosphere or from water “There in nothing that is fortuitous or to no purpose in Existence”. So that answers your question: ‘Must it (a planet) be currently inhabited or could it have been in the past to be counted?’
“Must life originate indigenously in a planetary system for it to be counted?” My view is that life originates in all planetary systems ,GIVEN TIME . And this view was there even before I came across the Dhamma. The Dhamma has embellished it, one may say.
“What is the obstacle that prohibits a Buddha from travelling beyond this set of 10k systems?” Whoever said that the Buddha is prohibited? It has been stated that He could travel anywhere among the world-systems – now there is a sutta where the Buddha states that the number of beings he actually reaches is insignificant and can be compared as the dust on his fingernail to the great Earth, and that displeased Him without measure! I see it as a question of sheer numbers: if reaching all of the vast (and yet not infinite) number of beings in this one 1,000-star grouping is already a self-confessed impossible task for even a Buddha in his mere 45-year Ministry, why would He consider reaching beings in other star-groupings? So it is not that the Buddha is prohibited, either by way of limitation to his powers or by any one else, to travel beyond this 1,000-star system (taken as a unit). It is just that it would be superfluous to do so, seeing that few of even the beings ‘at home’ themselves can be reached, as much as He would have liked to reach many more of them out of His infinite Compassion. Besides the fact that most beings, at any time, are in the apayas, even those connected with Earth, so they too are ‘out of reach’ And as to other regions ,other realized Ones will take care of those. There are Buddhas aplenty, an infinite number of them.
The last point you mention becomes relevant here:
“If he wished, Ānanda, a Realized One could make his voice heard throughout a galactic supercluster, or AS FAR AS HE WANTS.”. So, certainly, he is in no way prohibited. Further along in the sutta (AN 3.80) it goes: ” “First, Ānanda, a Realized One would fill the galactic supercluster with light….When sentient beings saw the light, the Realized One would project his call so that they’d hear the sound.”
“So a Buddha from one dasasahassi lokadhātu could communicate with a Buddha from another dasasahassi lokadhātu, or any being, anywhere in the universe, that is receptive.” No Buddha communicates with another Buddha, because to what purpose? Who will help whom on the Path? What Path? Both are already enlightened. As to ‘communication with other beings anywhere in the universe’ he COULD do that ,but He does not, for the reason given above.
Enough said. Perhaps more than enough- the range and scope of Buddhas is one of the four imponderables. AN 4.77: “Monks, these four things are unthinkable. They should not be thought about, and anyone who tries to think about them will go mad or get frustrated”