If the Feliway works it buys you time for them to learn to live with each other. You'll have to keep it up for a couple of weeks at least, maybe longer. Then you can try removing it to see what happens. When it does work for folks, it's pretty much concidered a miracle.
Sometimes you can get away with using it for a short period while relationships settle, sometimes you just have to keep using it. There's only one way to find out which it will be in your case.
Definitely don't lock them in a room together.
Think of it in terms of that very annoying, loud mouthed, nattering and nattering away old lady who is ALWAYS in front of you in the queue at the post office/chemist/doctor's surgery. Given time, enough space (and perhaps something soothing and calming in a glass
) you
may forgive her for all the hours of your life she's wasted. But how long would she last if you stone cold sober and were locked in a broken lift with her.
Rule out anything to do with hormones or gender. Cats size each other up by smell. Once they've been neutered, they are no longer sending out those signals.
The only ways scents can be a factor is if she is socialising with a full Tom and
his scent is getting on to her. If she's spayed though, a full Tom would have no interest in her.
I still think there's a smell she's bringing in that's upsetting him OR she could be a nasty bully on the quiet who beats him up if she can corner him when you're not looking.