I'm very sorry to hear about your problems.
As I'm sure your vet will tell you, there are many different strains of flu. Some are deadly and produce horrific mouth ulcers. Others are not so dramatic and easier to fight off, so it all depends on which strain of flu they have contracted. A vaccine can't cover all known types so the drugs companies tend to focus on the more dangerous ones.
Any kind of flu spreads like wildfire in cats that live in close quarters. So you're lot have spread it around themselves... it might have only taken one germ from the tom to land on one of yours and that's enough. The good news is that if this tom appears healthy he's fought it off, and although he remains a carrier and passes it on, the chances for any cat he comes across is good.
I would avoid the RSPCA as they will PTS him if he's suspected of carrying a flu strain. He need to be bought in and sorted, then rehomed as solo occupant in a stable home where he's not going to get stressed as it's the stress which makes them shed the virus.
Your own cats should never be allowed to mix with others again as the chances are one or more of them are now carriers themselves, and if they shed the virus they will infect other cats too.
So there's lots to be hopeful about. Think of cat flu as being similar to the human Herpes virus. Some simple precautions are needed.