(1) The biceps femoris doesn’t seem to be worked preferentially by eccentric training, no. When researchers have tested eccentric leg curls, it tends to be the semitendinosus that is most strongly worked. Leaving aside the nature of the exercise as a knee flexion movement, we might expect this based on the length-tension relationship (eccentrics operate in a very similar way to long ranges of motion, insofar as they load passive elements inside the muscle fibers).

https://www.instagram.com/p/BrPiTh1gdcj/

(2) Anatomically, the biceps femoris seems to divide more logically into deep and superficial regions, but that doesn’t stop researchers from analyzing the muscle in proximal and distal regions as well, according to variations in muscle architecture or fiber type. We can arbitrarily divide a muscle into as many regions as we like, but some divisions have more validity than others.

(3) Active insufficiency is where the muscle fibers are too short for active force production to occur through crossbridge overlap. So the bicep femoris might be the hamstrings muscle that is *least* effectively trained during a glute bridge. Although none of the hamstrings are probably trained that well during glute bridges, hence the name of the exercise.