Okay so I've read that moving a lot with graves on it causes corruption, which bothers me because I love using the House of Fallen Trees and Gothier Green Lawns in my custom neighborhood. So I downloaded a clean version of the Downtown, added both lots from there to my lots & houses bin and then placed them in my neighborhood, then ran the HoodChecker through it before starting gameplay. I am wondering if this would still cause corruption, since I got the lots from a clean version of Downtown and cleaned the new hood after they are placed? I really like having the Tricou graves on the lots since it adds story to my gloomy neighborhood but I'm not so sure this will avoid corruption...

Yes, moving lots with graves on them into the lot bin causes corruption.

The thing to remember is that a grave is a sim. Moving a sim to the lot bin deletes the sim from the neighborhood, which corrupts the neighborhood; moving the lot with the sim on it into a second neighborhood then corrupts the second neighborhood. Basically the sim's data are shredded - it loses all its relationships, and also leaves some data behind in the old neighborhood, and then arrives with incomplete data in the new neighborhood. Therefore, it doesn't matter how clean the downtown you added is, any more than it matters that you clean a dish before a shatter it.

If you want the House of Fallen Trees and the Tricou ghosts in your custom neighborhood, rather than in a downtown attached to your custom neighborhood, you need to add the downtown to the custom neighborhood, move somebody into the House of Fallen Trees, and move the graves to a graveyard in your main hood using the move graves feature. You can then move the (uninhabited) House of Fallen Trees to the lot bin and move it to your custom neighborhood, though you may want to scrub it of references with SimPE first. It would certainly be prudent.

The Tricou ghosts in Gothier Green Lawns are trickier, but I believe it can be safely done with a utility called the HoodChecker. Make sure you prepare a lot for the graves to arrive at ahead of time. I am not very familiar with the Move Graves function of the HoodChecker, but I bet somebody here will be able to explain it.

If you don't mind not having the lots, you could also add an empty, custom downtown, which adds the Tricou data without the graves, and use the teleporter and the testingcheats to spawn the graves; but if you are attached to the lots this won't do you much good, so I will not go into that procedure at this time. I always forget it in between times I have to do it.

If you just want a gloomy ghostly atmosphere then I'd take a clean graveless copy of the House of Fallen Trees for your custom neighbourhood and then kill some townies on it.