CC, well done for looking after Seb in this way.
I know what you are feeling, as we went through the same thing with Tatty in December last year. At least at that time, when the CP vet told us that Tatty was FIV+, even though they implied that they thought that we should have him PTS, because we told them we were prepared to keep him as an indoor cat, separate to our existing gang, they let us take him back home.
We were worried about our existing cats, bacause they had been associating with Tatty for about a year before we found out about his FIV status, and he was a very different cat to your Seb. He was so very scared of people to begin with that he would lash out with his claws and bite if you got too close to him. However, it took a while, but about two or three months after being neutered, he became a totally different cat. He loves the attention of people now, and the only problem that we have is that he hates being picked up. We carried out introductions to our non-FIV cats very carefully and very slowly after Tats had settled down, and we now trust him completely with Gizzy, Suki and Sweepy, but not with Tommy. However, because I am around all day I can ensure that Tommy and Tatty can either aviod each other, or get to their respective refuges safely when necessary.
I understand your worries, but I can only say that we have never for one instant regretted bringing Tatty into our family. It hasn't always been easy, but is has certainly been worth it. In fact, our latest waif and stray (as our vet has called Tabs) is currently at the vet today for his neutering and dental treatment, and his Glasgow FIV test came back as positive.
I'm so pleased to hear that you are going ahead with the neutering. If you do go ahead with the FIV test on Wednesday, then I hope this comes back as negative.