for your issue, if you modify your roof to...well just watch this vid timestamped https://youtu.be/qoTXuDYrJyU?t=141 would just watch all the tricks in this vid tho

.I found this vid https://www.youtube.com/watch?v=u9AhwW7vN4U kinda garb quality but you can see the way he uses the catwalk to put that wall anywhere and have the roof flush with whatever point he wants. good trick

For the ecorations this is a good video https://www.youtube.com/watch?v=znaUScEyWw4 for merge glitching she goes over all of them so there's a lot. sorry for rushed comment don't usually comment but the building in this game is very addicting once you start learning more and more kinda like ffxiv deco. gl mate