...the questionable ethics on why the aliens would pick a planet that's so inhospitable for a human being. And if they have the technology to build a sustainable complex in such environments, why not do something simpler like put it on a moon where he can be spotted and potentially rescued?
I think this assumes that the aliens would be only just
above our TNG heroes in terms of technology, and only just
alien enough not to count as bumpheads. This "solid Neptune" probably offered them no greater challenge than some airless moon or lush jungle or Class Y hot hellhole, so they never "picked" a planet - they merely put Richey down on the planet that was already there. And if they were alien enough not to recognize the fundaments of Richey's existence (and, this given, not to hang around until they could interview the poor sap for better intel), they would not think in terms of fellow beings coming to rescue.
The very fact that they departed without bothering to communicate with Richey suggests they came from the category of classic aliens somewhat underrepresented in Star Trek: those who consider humanlike creatures mere uninteresting animals. Richey was probably saved by exercising the same relative effort a human gives to a drying earthworm when lifting it off the sidewalk and dropping it on the grass on the divider, and forgotten in an eyeblink.
One has to wonder why purchasing the casino allows the crew to leave the hotel in the first place.
My bet is that the setup allows anybody to leave as soon as he displays the intent to - that is, as soon as he figures out the way it is going to happen. Any and every way is possible and right, but you have to make the effort. Data chose an exit based on concluding the story; Riker could have chosen an exit based on seducing Rita, and Worf could have chosen an exit based on killing everybody twice and eating their hearts both times, and all of these would have worked. It's just that poor Richey never realized he had to have this sort of perverse faith on what he was doing.
Again, the aliens would think of Richey as a worm: they leave the lid open, and don't stop to ponder whether the worm really can crawl up the steep sides of the jar.
I think that's fair for the writers and the story - accepting that the aliens are alien, with alien motivations. They aren't slightly advanced and very snobby Metrons or Organians, but something that really will
remain a mystery forever. And all because of an issue that was at the very heart of the story: they have a fundamental problem communicating with us, and we have a fundamental problem communicating with them, because some gaps are just too wide to bridge.