There are controversies over Ivan II, but he at least initialy was viewed by other governments as King of the Bulgarians and Vlachs. However, you are correct to read the first part of my post as suggesting that a relatively modern personal union between Romania and Bulgaria is an impossibility. The two states cannot even agree on the ethnicity and composition of the Second Bulgarian Empire, but they are supposed to share a single monarch?
Are you sure this controversy existed in the late XIX century ? I agree that today Romanians and Bulgarians have their differences, (I even hated the Bulgarians for one week after they equalized from 2-0 against Romania, in Constanta) but I don't think there were many arguments back then (except Dobrogea maybe).
This post is odd because you've essentially explained why the personal union cannot happen. Bulgaria and Romania are separate countries with distinct cultures and history that cannot agree on whether or not a Vlach was always a Vlach or a Bulgarian was always a Bulgarian. In Norway and Sweden you have two historical states with strong links of being bound together that share similar legal and belief systems traditionally, and communicate in quite similar languages.
Same as the first part of my post. I don't think what you describe here was completely true in the XIX century. Yes, Romania and Bulgaria are separate countries with separate cultures, no one argues that, but so are Norway and Sweden. Also the history of the two countries in the past five centuries, although distinct, had some common points. I agree the links are not so strong as the links between Norway and Sweden, but it's not like were talking about Egypt and Brazil here.
Once again, I have to specify, I'm not stating that such an union would be bullet proof and would last 1000 years, but it's definetely not impossible.