Is it considered a sin to repeatedly watch educational video clips that may contain images of nude women, knowing that there are alternatives free of Haram (unlawful) content, and that the user closes the clip immediately upon their appearance?

There is no harm in perusing useful topics even with the expectation of encountering forbidden images. If you come across them, avert your gaze from them, and you will not be held accountable for the first glance. However, if you know that perusing the website will inevitably lead to seeing lewd images, then it is not permissible to open it, because blocking the door to the forbidden is obligatory. You must stay away from what leads to forbidden things and restrict yourself to lectures free from prohibitions. Allah has commanded lowering the gaze; the first glance is for you, but the second is not for you. Sustaining a gaze upon what Allah has forbidden inherits blindness of the heart, weakness of insight, and falling into vice.