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[Infinity War] Spoilers: Let's discuss the nitty-gritty of Thanos' plan

So Thanos states that he wants to kill/erase half the population of the universe, and he wants it to be "at random. Dispassionate, fair to rich and poor alike." 

As people have pointed out, there are at least a couple of exceptions to true random: He's not culling himself, and he promises to let Stark live. (It's possible that he left his fate up to the algorithm, though he has post-snapping plans, and it's possible he meant he wouldn't kill Stark right then but that the snap might do him in anyway. The oral agreement is light on details.)

Questions:

Is the "half" measured on a universe-level scale, or by planet, or population, or species? 

If it's a universe-level "half," does that mean that some planets or civilizations were hit unexpectedly heavily, and that some were relatively lucky? 

If it's a universe-level "half," what's the likelihood that any given population is completely unaffected, or completely wiped out?

If it's species-level, are unique entities immune or doomed or determined on an individual basis? Since Thanos' goal is his version of mercy, it seems wrong to let the snap eradicate an entire population. 

How long did he have to think about and formulate the details of this "wish," or does he just say "half the universe's population" and be done with it?

Is he just mistaken about the lack of pain? He claims he's being super merciful, yet clearly people who are erased know that it's happening, and feel pain and fear. And he doesn't care about the effects — seems like another large chunk of any population will be killed by stuff like accidents triggered when people disappear. (See: Cars crashing, helicopters dropping out of the sky, etc.)

Basically: What kind of random are we talking about, and how might that reshape the universe once he's done?

(I'd love it if someone in the next movie checks in on Gamora's home planet and finds that they're materially well-off but damaged or depressed on a sociological scale because of the mass casualties.)
gpt
I don't think getting Snapped is painful, just scary and feels weird.

Since he's trying to balance out resource consumption, it wouldn't make sense to eradicate on a civilization or population level, taking entire worlds while leaving others untouched.  It's probably at the individual level, like picking every other person in a lineup.

None of the plan makes sense.  Resources have always been finite and, if you include stars, most civilizations are only using a fraction of a fraction of a percent of matter and energy out there.

It's also silly because it assumes every species in the entire universe is at the same level of development and using too many resources for its own world.  Clearly, though, there are primitive, subsistence-level aliens as well as advanced societies with things like matter replicators.

Thanos is insane.  Sadly that is usually the type of person who feels like they should make decisions for everyone else.