You absolutely made the right choice at the right time. Don't ever question it.
I tried everything when Cinder was sick. This was years ago and they were even less knowledgable about FIP than now. He was not quite a year old. I took him to two different clinics and was waiting on tests when he passed away. That morning he was breathing funny also and was very unsteady on his feet. You keep waiting thinking that once they know what it is you can do something.
I checked on him at noon and then came home early from work to find him dead. I'm sure he suffocated and suffered before he died. This is what would have happed to Mons if you hadn't helped him. There really is no happy ending where FIP is concerned, only the kindness to see that they do not suffer. RIP Mons.