1. The walkthrough is not always reliable. If you aren't in a relationship with Evelyn, then you cannot have sex with her while she's with Callie. If you haven't had sex with Callie, then you can join in, but only to have sex with Evelyn. To make the threesome path work, you have to have had sex with both of them before you try to join in with them.
2. No. Invade mind and remove inhibitions are necessary only for a few of the romance paths in the game. And you never actually use magic to make someone become attracted to you (except for making Callie be bisexual instead of lesbian). What Belle has to say about the use of the spell and what you actually use the spell for in the course of the game are quite different.
3. After the first week, each time you try to talk to Thaddeus while Evelyn's portrait is next to his on the map screen, you trigger an NTR scene. The first scene is them kissing, the second is oral sex, the third is anal sex. Each time, you're given a choice of watching them or interrupting them. Interrupting stops the Evelyn-Thaddeus thing permanently. IE, if you interrupt the first time, the relationship is halted at the kissing stage before anything really happens.
Interrupting them cuts off the money Evelyn gives you every Friday. Not interrupting, or not ever seeing them kiss at all, makes Evelyn *much* harder to seduce. What I do is not look in on them until I no longer need the weekly income, and then interrupt them. If you prefer, you can interrupt them at the very first opportunity during week 2 - you'll only need to do one more shift with Callie per week to make up the deficit, and money becomes pretty irrelevant once you've paid for all the training from the Crone and Samarra anyway.
