First off, let me say that I am sorry you had these results. I don't know the precise cause, but I can provide some background on glass manufacturing that might help you avoid these results in the future.
The Bullseye cullet that you used is graded as Bullseye Compatible F grade. In a nutshell, we never describe our compatible glasses as "COE (coefficient of expansion) 90", because that would not be accurate. Our tests (and the tests of other manufacturers of fusing glass to our knowledge) do not actually test for COE. Our test is more basic: it shows us whether each small lot of glass will work successfully with any other Bullseye Compatible glass in the kiln.
Our glass is designed to be compatible with all other Bullseye Compatible glasses. We don't recommend that Bullseye glasses be combined with glasses from other manufacturers, because we can't know or speak to the consistency of testing protocols that are not conducted under our roof, or under our supervision. At this time, Bullseye is the only company that makes Bullseye Compatible glass.
Some dealers carry multiple brands of glass, and describe some of these as "COE 90", but we find that use of the term "COE 90" to be less about science and more about sales-speak. Sometimes using the glass of two manufacturers together works. And sometimes it doesn't. So when you mix the glass of two unrelated manufacturers, the factory testing that you paid for to both the manufacturers is no longer valid, and you need to do your own testing to ensure good results.
For a more detailed report on this very topic, you might be interested in reading this article by Bullseye's owner, Dan Schwoerer: http://www.bullseyeglass.com/pdf/techno ... tes_03.pdf
Re your other questions, I can't speak to the casting quality of Uroboros glasses or what the Uroboros guy shoveled into your bucket.
Let us know if you decide to cast your project again, with a single brand of cullet!