In 2017, Thomas Wayment and Haley Wilson-Lemmon published A Recently Recovered Source: Rethinking Joseph Smith's Bible Translation which announced that they had discovered a number of parallels between the Joseph Smith Translation and Adam Clarke's Commentary.
The full article was finally published in June 2020 as a chapter in Producing Ancient Scripture: Joseph Smith's Translation Projects in the Development of Mormon Christianity (You were looking for Kindle version, which is $40)
No, it is not available online. I haven't read it the actual article either, but I've studied a bunch of what he has said on the topic, so maybe I can help answer your questions. A few things I'd recommend:
I feel like the critic's argument is largely overstated. Sometimes it is implied that Clarke's commentary is itself a translation, but it is not. It is a commentary. Also it is implied that the entire thing was copied, but Thomas Wayment estimated about 5% of the time.
Thomas Wayment points to the instruction in D&C 88:118 to "seek ye out of the best books words of wisdom; seek learning, even by study and also by faith." I would add to that that there is the idea in revelation that we are to "study it out" in our mind, and this may have been part of the revelatory process.
But even though there's nothing inherently wrong with it, I agree with Kent P. Jackson that the evidence for using Adam Clarke's commentary doesn't seem that strong.
What portion of the JST changes were taken from the Clarke Commentary?
In his interview with Laura Hales, he said there were "about 200 to 300" parallels. There are 3,410 verses in the published Inspired Version are different from the King James Version, which would give around 7%. In his Journal of Mormon History article, Thomas Wayment said
Although an exact percentage of how often Smith drew upon Clarke is difficult to generate, my own research has demonstrated that in the Gospel of Matthew, for example, Smith engaged Clarke for 36 verses of approximately 639 verses that he altered in his revision. If that percentage holds throughout the process, then it is possible to suggest that Smith relied on Clarke about 5 percent of the time.
Reciprocally, what portion of the Clarke commentary was not included in the JST.
To answer this precisely, you would have to determine how many changes Clarke suggested. Except it's just a commentary, he doesn't really suggest changes, he is largely just explaining how to interpret the scripture. Imprecisely, given only 200-300 parallels, just a few words each, while Clarke's commentary is six volumes, each around 900 pages, the commentary typically taking up at least half of each page (KJV reproduced at the top, commentary on bottom)—I think it's fair to say that the percentage was pretty close to zero.
What were the nature of the changes incorporated by Clarke--typographic? stylistic clarifications? translation errors? substantive doctrinal changes?
I would say probably mostly clarifications, but some translation errors.
What were the nature of the changes that did not come from Clarke?
There are a lot of clarifications here, too. Most notably though are larger additions, particularly in Genesis.
Of the Clarke sourced changes that track to the JST, how closely do they track? If not identical, in what way were those changes altered from Clarke?
Since Clarke's commentary is a commentary, the way that the parallels track is that somewhere in the commentary for the verse is an explanation for what it means, and then the Joseph Smith Translation shows one or more changed words that makes the explanation clear.
Kent P. Jackson's article talks about how closely they match, but he focuses more on how else Joseph may have come up with the same change. But there is this Quora answer where at the bottom are quotes the KJV, JST, and Clarke's commentary so you can compare yourself, mostly without commentary. (I think Jackson's comments in his article are very helpful though, as he provides a greater context.)