I assume the difference is the cotton vs the poly of the coats and clark. My last quilting as one that didn't come together well, but I quilted it for more practice, and to see how the dream black poly batting worked. Hated it!! The lint from that was pretty crazy. I normally use the quilters dream 100% cotton and it's normally ok. But with switching to the cotton thread, it's noticeably worse, but the stitches seem much better, especially on the bobbin side.
As for the bobbin case, is that website (sewing parts online) ok to use? I see some bad reviews about them, and some say the will report bad reviews to get them deleted. And that when you click on genuine parts they tend to ship the knock-offs instead. I would just order it from my repair guy, but he's 45+mins away each way, and with my health, I don't travel well.