I summed up my opinion of the score estimator in another thread recently 
, so I will simply link that: Score Estimate Bug?
And where do you get this idea that unfinished boards should be scored? What if an unsettled area makes the difference between a black win and a white win? It is impossible to decide and also highly unusual.
I don’t know how dead groups are pre-selected, actually. My unsubstantiated gut feeling is that it is not random. I guess it is an interesting question for the developers , but also practically irrelevant.
Whichever method, it only needs to be “good enough” to cover the most common cases.
I got the score wrong twice. It is B+28.5, like you said all along 
Then why is there such a weird difference to the territory score of B+30.5, when the two methods are supposed to be basically equal? This threw me off until I realized that this is a handicap game; black has one extra stone at the beginning of the game (in addition to the last move).