I'm very sorry for Dani's passing. We've had similar experience with a feral (probably a dumpee and not born in the wild) that we fed all summer outside, live trapped in Fall and then he turned out to have wet FIP and we had him euthanized. I know how attached you can get in what seems like seconds.
Unless you go to another Vet for a necropsy you are never going to know for sure what killed Dani. Maybe he'd already got a small dose of the street poison. Maybe the whammy of all his vaccinations and the sedation at once was too much for his system. I'm not sure what Felocell is but I have to separate the rabies from all other vaccinations and do them a couple of weeks apart. And I've not had a cat sedated to get a chip but then I haven't chipped my really wild ones.