Question: The burial vault which Matt and Karen hide in has the name "Edmond Cleary Cass" on the front of it, shown pretty. Does that name have any significance to the show or Marvel?
Answer: I did a little research too. Interestingly enough I found a Friar named "Edmund Cass" who as far as I could tell is still alive and well. I wonder if that was just a coincidence.
Question: Does Kat ever wise up again to Kara's secret? That and why did she (Flockhart) leave the show only to come back?
Answer: Yes, s02e22. As Kara leaves her office, she said "go get them, Supergirl" to herself. After season 1, the show switched from a CBS production to a CW production and filming moved from L.A. to Vancouver. Flockhart isn't really a fan of traveling to shoot, and didn't want to be away from her family for so long.
Chosen answer: After some research, I can say that no, Cass is not a character of Marvel, nor is he present in the Netflix show. As far as I can tell, it's either a real person, or a random name chosen for the coffin, most likely the former. In another season of Daredevil, the show utilized a real cemetery displaying real names in the tombstones.