Glad to know you liked it!
1 - You can know about Melissa by talking to the smoking kid with the leather jacket in the Blair Leamnach College. She is the girl some people are talking about as being the victim of sexual abuse. She is mentioned is later mentioned at the Sweet Revenge, during your meeting with Ewan(she is one of his victims) and a few more locations, I believe. There is also a certain scenario where you can find her body by the Seannloch while waiting for the day of the Ritual, but it depends on many things.
We recently fixed a bug where she was being mentioned in Ewan’s notebook files without being introduced first, but I don’t know if the game t has been updated yet.
2 - There is no way to enter Gyllenhall Mansion for your second time there, as they found out about you being there before and are put the entire place under surveillance.
3 - 19 is the highest deduction score you can get.
4 - That depends. There are many, many branches in that segment of the game. Some scenarios make it so that the offerings all die, for example. Another(perhaps the same, I don’t recall right now) makes it so that you can kill both the cultist leader and the child when they try to flee. Someone posted a guide on the game thread recently, perhaps it is worth to check it out.