Why do the Eastern Orthodox consider the Quninsext (Fifth-Sixth) Council, also called the Council of Trullo, a part of the Ecumenical Councils?
Because that has always been the tradition in the East, and, in the West, Trullo has enjoyed similar or close-to-similar recognition at various times. For example, according to the Liber Pontificalis
, Pope Constantine I provisionally accepted the canons of Trullo in 710; and near the end of the same century, Hadrian I referred to canon 82 of Trullo as a canon of the "sexta sancta Synodo." Then, of course, there is Hadrian's famous letter to the Patriarch of Constantinople, in which Hadrian accepts all of Trullo's canons as belonging to the Sixth council.
Much later on, Cardinal Humbert, ever irascible, claimed the Trullan canons were never and would never be accepted in the West, but, not long thereafter, Gratian included a good number of them in his collection, and the original, official, Roman Catholic printing of the decrees of the Sixth Holy Council, published in Paris in 1540, include Trullo's canons.
I could go on. In particular, in the last 30 years, there have been many official Papal documents, e.g. even an Apostolic Constitution, that recognize the ecumenicity of Trullo. As Pope John Paul II wrote in Sacri Canones
, Canon 2 of the Seventh Ecumenical Council demands that all Christians recognize Trullo's canons. Incidentally, logic kind of demanded that he and the Roman church recognize this fact, since they were promulgating an Eastern Code of Canon Law that depended in parts directly on Trullo.
It looks like, at the time of their office, the West was not only unrepresented, but a fake representative was present.
Not so. I think you are confused because you are depending on the Internet. Here's the problem with Schaff (editor of the source you originally quoted): While his work is widely available online since it is no longer under copyright, that means it is outdated -- in his case, by well over a century! In addition to age, it also suffers from some pervasive biases, comes from a polemical era, and contains no small number of straight-out errors.
The East is as guilty to coercion? That's not the usual picture.
You're thinking like a modern here. Justinian II promulgated the Trullan canons as part of the Empire's official legal corpus, binding in East and West. So, any resistance to Trullo was really just a footnote in a much larger, political battle between the Roman Emperor in Constantinople and the various "barbarian" Kings and nobles in Italy.