Question 8.1: indeed Recall is here important, so you look for the better system at high recall, which clearly is S2 (compare for instance precision scores for Recall 0.6; or look at this picture :
Saying that "the two systems have same recall" is not really meaningfull as recall is increased incrementally on any system evaluated in order to plot the Precision-Recall curve...
Question 9.2: sure steps 4 to 6 look like K-means and are unsupervised, but steps 1 to 3 are indeed supervised (definition of topic t), thus the whole is hybrid.
