In the order they’ve come up…
Teachers of yoga should not “force” students, ever. It is not the role, task, job, mission or mandate when the person is committed to teaching AND the subject is Yoga (or in this case asana).
Student who are not prepared physiologically for forward bends should not be doing them and certainly should not be doing them full bore. The flexion of the knee joint in seated forward bends is the student’s “out”. For this reason, obviously, seated forward bends are a bit more demanding than standing ones since the knees can be flexed more easily in standing.
Of course it is of note that hamstrings do not get fully stretched when the knees are bent in either standing or seated forward bends. For a student that has the opening already there, straightening is okay and perhaps even warranted. For a student without said opening it is risky.
It should also be noted that the effects of asana are equally available to the student trying to straighten the legs as they are to the student straightening them. The effort or attempt is just as beneficial as the doing.
At the most rudimentary level, moving one end of a muscle away from its other end is “stretching”. Stretching muscles can be uncomfortable. This discomfort is heightened when a) the muscle has knots and/or tension and b) when the stretching goes beyond a certain range of motion. We are trying to work out “a” and not obliterate “b”.
You ask about enduring pain. Pain is not discomfort. They are different and I see no point in a beginning student (for those in the first ten years of practice) “enduring pain”. It does not lead to a more profound practice. However…the purpose of Yoga (not asana) is to bring more light into the physical form and unify the mind, body, and spirit. In that respect the vessel (the body) which holds the force (light) does require some opening. How much and how to go about it are other questions which should be explored by the student with the guidance of a sound teacher. Not explored by the teacher with the body of a sound student.