In both cases NI' is natural and lowered "by the author". There is no any attraction. To be exact, NI' is lowered due to the melodic phrase, which cannot be executed with NI' sharp. In many cases of the B plagal mode you will not see a diatonic sign on DI ("pthora") in order to execute high NI' natural. It's known by the melodic phrase and the "pthora" may be omitted, mainly in older texts. In new editions the writers try to put the "pthora", in order to be accurate in the intervals.
By the way, the mode is not B plagal but B "heirmologic" ("of the heirmologion"). This mode is a mixture of B and B plagal. When the melody shows two voices upper the base (suppose PA as base, GA is dominant), the mode is (or tends to be) soft chromatic. When the melody shows three voices upper the base (PA=>DI), the mode tends to be hard chromatic. So the beauty of this mode is the alternation of soft and hard chromatic scale. Because of the frequency of the alternation, the alternation of the intervals has also to be normal. You have to slightly go from hard to soft chromatic and vice versa. But this is not difficult: you simply don't think of the theory and your intervals become natural!
Another comment: at the first example, the NI of the phrase "to di e lon ti tha las san" is natural and not hardly sharp, as it is executed.