Again, I don't know anything about that side of things, but Fleabusters offers a nematode treatment for your yard. Supposed to be natural, but I've never done that because the Fleabusters powder inside the house we used worked. Couldn't even tell you what a nematode is. Not trying to say that's the only solution, either. Don't even know if treating your yard really works. I'm always worried about using anything with too many chemicals, though. Feel like we pollute our environment enough already. The only thing I know is what I have done in the past. This borate powder works and supposed to be safer. (I'm not affiliated with Fleabusters, I promise! I will also say that now that I've stuck my neck out to recommend something that I'll worry forever that in a few years they'll discover that borate powder causes cancer or something--and I've killed someone with my bad advice!)
I'm so sorry that you and Tegwen are going through this, Siwsan. Wish I could help. I finally joined this forum because people here really care about each other. There are a few times I've read people's disagreements on DU, but people here are kind to each other, and try to be of use to each other instead of tearing each other down. The Admin has great safeguards in place to protect everyone, (thank you, Admin!) I lurked a long time on DU before deciding to participate in a forum again because of the horrible flame wars and just downright ugliness I saw on other forums. Broke my heart.
Also would like for you to have a cosmic "win" right now, Siwsan. You've been through so much and you put so much of your heart into your kidcats. Will be thinking of you, Tegwen, and your indoor and outdoor kidcats. Hugs!
P.S. Just asked my husband and unfortunately he can't remember what skin flea treatment the vet prescribed Seb either. I got rid of all the paperwork on Seb last December, so can't look it up. Don't know how long they keep paperwork on animals at our vet's, so not sure they would even have Seb on file anymore. Was nearly two years ago, Seb died. You probably couldn't use the flea treatment on a kitten anyway. Sorry again I can't help.