As far as the main room, I didn't do much here because too many Dawnguard quests are tied to objects here. It is wise to refrain from modding high use areas by the base game in my modding experience. Minimize re-works and mistakes by making your own areas.

This mod has had over 2 years of modifications made to it. The only way to be sure you don't mess something up is to make a duplicate of anything you make and use the utility within the creation tool that compares vanilla with changes.

Thanks for your response, honestly I was genuinely curious, as I was hesitating to put in a mod that simply added a spell buff to Harkon's room and Valerica's room, and I hadn't previously considered whether I could give some kind of problems (Also I haven't even started the DLC to the point of reaching the castle yet, so I don't know the exact changes). And I thought that more people would experience the same thing and I would have trial and error to see if there were any problems.

To be honest, trial and error and lots of research online to see how to fix things is how you learn the creation kit. The documentation from the maker garbage in layman's terms. Thankfully, there is a ton so you can do almost anything you want. And there are plenty of authors that if you PM them and ask for help on coding, they probably will help you. I added every single person and resource to the mod's landing page and gave them credit. I didn't do this alone, no way I could have.

Good luck with your modding.

PS
If you ever release your own mods, a little advice, don't be a slave to that forum. Don't answer stupid questions. Don't answer questions they could have easily googled. 99.9% of subscribers are selfish and lazy. Only 2-3% of subscribers click the upvote to show appreciation and 90% of your chats will be from complainers telling you what you *should have done.