I've used the spray. I live in a 3 story house and there is no way I would be able to afford the amount of plug ins I would need to cover the house.
All I can say is, it isn't a miracle worker, but it does have an effect.
My experience with it came to show itself around feeding time. Usually, it is quite raucous around here, especially at breakfast, so I sprayed a bit around the kitchen and me. Let the alcohol dry off about 10 minutes before exposing the cats to it.
It seemed like everyone was in slow motion LOL. It was calm(er) and there was a lot of sniffing going on at first. As for what I was wanting to see, if I could gain more trust, I can't say it did all that much.
I guess the best way to explain it is, it helps keep them from being riled up. And let me stress it only helps
A constantly meowing cat, it might help some, but that depends on why the cat is meowing. (They only "meow" to communicate with us humans) You may want to investigate what is making her meow.
The one knocking things over, like MowMow said, it's not going to help. Some cats love that game, others couldn't care less, but that's more in the personality then a behavior issue.
If you use the spray you do have the flexibility of spraying areas were it is needed and there is a lot less "wait" time to see if it does anything at all. That way you could tell if it was worth buying a diffuser.
Good luck with your kitties