I found this paper "Cajetan and Luther: Revisiting the Roots of a Schism" written by Dr. Adam Cooper, a Lutheran pastor turned Catholic, currently a faculty at John Paul II Institute for Marriage and Family which delves into how Cajetan became very alarmed as he detected (with prophetic insight) the far reaching consequences of Luther's view during the October 1518 conversations. The Extravagante Papal teaching mentioned by the pamphlet is covered in this academic paper along with technical discussion of the treasury of merits. The purpose of the paper is precisely what you are looking for.
From the Introduction:
Like most non-specialist students of the Reformation, my access to the
exchange between Martin Luther and Cardinal Cajetan in the city of
Augsburg, October 1518, has by and large been limited to Luther’s own
account of the event, or else to secondary sources understandably
writing with a Protestant bias. Recently, however, I made it my
business to read Cajetan himself, who, like other Catholic
polemicists, has suffered virtual demonisation in Lutheran folklore.
This essay is an attempt to restore some balance to the popular
picture of Luther and Cajetan’s exchange in Augsburg by way of an
examination of the latter’s own theology as manifest in the documents
he wrote at that occasion.
My aim is to show that Cajetan’s negative impression of Luther’s
theology – taken on its own terms - was not as illfounded, irrational,
or unbiblical as is often implied. On the contrary, it will become
clear that in Cajetan’s estimation, Luther’s theology represented a
radical shift away from mainstream Christian doctrine, and a
capitulation to the subjectivist, humanistic tendencies of the
nominalist school of philosophy.
In reckoning as much, Cajetan may well have been mistaken. I shall
leave that judgement to the reader. Yet it is only by grasping the
Cardinal’s deep-seated intuition that Luther’s protestations, embodied
in the Ninety-five Theses, threatened the very foundations of a truly
efficacious sacramental theology, that we can, to my mind, make sense
of the ecclesial breach that followed and recognise, in humility, its
fundamentally tragic character. I therefore offer this study as one
small contribution to an ongoing conversation in the hope that, in due
course, such a breach may one day be righted and the Christ-like
prayer of the Roman Pontiff for ‘full and visible communion’ answered.
As for the full text of Luther's letter to George Spalatin mentioned in the pamphlet, this PDF from archive.org contains scholarly footnoted collection of Luther's letter (volume 48) covering those years (warning: large file, 26.2MB, high-DPI book scan, OCR-ed).