It's tough to lose them, especially at so young an age. I do hope that you find peace. Is there a way to get Franklin's brother tested to see if he really is a carrier? In terms of the depression and guilt, I know what you are going through. It does get better, especially as you explore the possibilities and come to realize you did everything reasonable. I know talking to the two vets who saw Scout, as well as other cat owners and the wonderful people on this board, has helped me enormously in understanding that we cannot control everything that our beloved pets may face.
Tonight, I tried to coax a stray kitten over to me that I saw at a fast food restaurant. He or she wouldn't come to me, but tomorrow I'm going back to see if I can be more successful. The best that we can do is know we loved our little furry children and pass that along to another one, not as a replacement but as a tribute. I know, in time, you will feel the same.