<i>"I have never heard of such behaviour after a neutering"</i>
Well, now you have : )
Actually today I am feeling cautiously hopeful that this will pass. The dog with the swollen scrotum seems to be improving, and the crazed desire of the other dog "seems" to be waning. I think what I found so frustrating is the lack of information about complications on neutering. It is almost like everyone is afraid to talk about it for fear it will be a blemish on the practice. Complications DO happen. It is better to talk about them so pet owners are prepared for them. The possible swelling of the scrotum for instance, was never mentioned. As far as my other dog suddenly becoming a sex craved Romeo, I do need to mention that he has the best "nose" of any dog I have ever owned. He tracks every step a bird takes in our yard. And so if the vet's theory was correct, he would very likely be able to catch the scent of something wrong with our other dog, and possibly misinterpret it.
Another theory I had was that the testosterone levels of "crazed dog" hadn't dropped yet, while the other dogs had and he was no longer sensing that he was another male. But when I asked the vet about testosterone and how long it took to get out of the system, he said instantly but then back tracked and said he didn't really know for sure. And then I read a comment from an online article from a vet who said that it can takes weeks or months. Information like this might be very helpful to know, but there doesn't seem to be a definitive answer.
Anyway, I thank you very much for your reply.