Referencing only the letter under discussion, there is no evidence in it that its authors are having "trouble with Amoris Laetitia and the mindset of those who really do like the recognition in AL's teaching that all situation are not reducible to a set of rules".

They have precised a question on a very concrete application of the document, i.e. an interpretation, on which some bishops have publicly taken one position, some bishops another position diametrically opposed to it, and on which the Holy Father has appeared to give assent first to one and then other position.

The application you propose of epikeia is not feasible because the law on the indissolubility of marriage does not meet the definition of an imperfect positive law imposed by human legislators but by Christ Himself in the Gospels, the perfect divine Legislator from whose edicts there is no higher authority to which to appeal.