As you mention, the word grail (graal, grael) as a common noun referred to the dish or plate used in meals served in courses (as the Last Supper, being the last legal Passover from a Medieval Christian perspective, certainly was). It is my understanding that the term grail was also applied to the Gradual for the reasons cited in your post.

Also, one of St. Bernard of Clairvaux’s most famous works was “The Steps of Humility,” a treatise on the 12 opposing virtues needed to counteract the 12 “Steps of Pride” enumerated by St. Benedict. I guess that means that Bernard’s “Steps of Humility” is the original 12-step program! In any case, Cistercian spirituality was vitally concerned with this “step-by-step” process.

It is believed by medieval scholars like P.M. Matarasso that the anonymous “Queste of the Holy Grail” was undoubtedly written by a Cistercian and is an allegorical rendering of Bernard’s “Steps.” The introduction to Matarasso’s translation refers to the quest for the grail as “the mysteries of the Eucharist revealed.”