Why ghosts are mostly recent:
Probably because people in ancient times counted 100000 residents as a megacity, rather than 10000000.
Why you mostly see them in dark places when sleepy:
This might be either a higher risk of hallucinations when the visual cortex tries to make sense of noise from low light and jumps to conclusions with a cycle of self reinforced belief (stronger with a scary story), or that quantum anomalies picked up by our brains are more observable when not flooded by the average of many random particles. One particle can jump back and forth in time, but on average they just go forward.
What we look like to ghosts:
First time ghosts are horrified when looking through our bodies. Instead of looking you in the eyes, they stare at your brain first, then your heart and lungs.
Walking through walls:
If a wall was present at the ghost's time of interaction, they have to make a little jump to avoid falling through the floor. I guess this applies no matter if it's a hallucination or someone out of phase, because it would be impossible for neither real nor imaginary laws of physics to tell where the wall ends and the floor begins when floors are at different heights on each side and made from the same material. Only another time where the wall didn't exist solves that paradox.