Para: Can sims learn the Medium skill without living in a haunted house?

There are no skill books, but you can go to skill courses for the Medium skill (with the Uni EP). Not sure about electives, I think there is no Medium elective.

On a sidenote, you don't need to maximize the skill to get the license. It's also in the reward store for 3,000 satisfaction points (and one of these days I want to see what happens if I buy it for a zero level, pretentious, investigator and start his career with no skill, but lots of self-confidence)