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

They can place and use the table but I think they build their skill much more slowly because they don't deal with actual paranormal beings. And you personally won't get to practice with actual ghosts

Yes, I did that because I wanted to see if it was possible. My second Sim I am playing as a Paranormal Investigator never lived in a Haunted House. I did, however, start her out in a Tiny Home. She had a seance table and just performed the 'Sense Spiritual Volatility' and 'Commune with the Departed' interactions while focussed and due to the buffs of the Tiny Home she skilled up pretty quickly and became a PI by buying the licence in the Rewards Store. Now that she has skilled up and made enough money as a PI, she is living in a very nice big home.

Awesome! I don't mind if the build gets slow, as long as it is possible it allows for more sims to be into these things without needing to go all the way.

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)

As has been said, you can build your medium skill without living in a haunted house.

However, you can't summon or meet Guidry unless you spend at least two nights in a haunted house, as he's not spawned until the second 9pm you're in said haunted house. I think, however, once you've done that, you can change the lot type back to normal and then summon him with the circle (once you've got your paranormal investigator's license.)

I suggest to use the:
Stats.Set_Skill_Level Minor_Medium 5

Also, the Paranormal Investigator is in the Aspirations Rewards list.

Thanks! I do use UI Cheats mod and can cheat any skill, but I want to play a progression, and I don't mind if it will take my sim years to get better at it. The only thing that might need cheating it to get the license for one sim, or I will not be able to play any sim as a PI until I'm done with say 20 rotations - that will be about 5 RL years from now LOL.