1

There's no such thing as a quick side trip in the Balkan region, unless if you fly :) In this case Bucharest-Zagreb-Dubrovnik. Or a night train to Belgrade, then a train to Bar, bus to Kotor and Dubrovnik.
Perhaps reconsider; spend a few days in the Romanian mountains (Sinaia), Transylvania (Sibiu, Sighisoara, Brasov) or Bulgaria instead, and save Croatia for another trip; it's easy to reach from Western Europe on several budget airlines.

2

I wouldn't say that it was daunting. In fact, I did a similar trip last fall. I traveled from Podgorica to Belgrade to Ljubljana to Zurich to Geneva by train spending a couple of nights in each place.

Whether it makes sense for you depends upon how much time you have and whether you want to make stops along the way. A great deal will also depend upon your comfort level as it might be hard to make transport reservations in advance.

I'm about the same age and was traveling alone. I stayed in single rooms but shared baths in hostels because I prefer no smoking settings and hostels usually don't permit smoking. Generally I find the staff very helpful in making recommendations and also I like having other people around but prefer some degree of privacy.

Ruth