So update on what appears to have happened: The game reverted to a pre-Dawnguard state, sort of. When I returned to Solitude for the first time after travelling to Solstheim for a little while, there was a master vampire and some thralls attacking the citizens. I fulfilled my thanely duties and killed them, while thinking that was odd because the vampire attacks were a thing they added in Dawnguard. I also noticed that I could still smith dragonbone weapons and arrows, which were also new to Dawnguard. Then one day on the streets in Whiterun this bro Durak shows up and asks me if I've ever heard of the Dawnguard. So its as if the game reset all everything to a state in which I had done nothing with the Dawnguard DLC content I guess. Its really weird, but hey I won't complain, its better than not being able to access the content at all.