Since she cant be trusted we have now had to lock her out of all the bedrooms which makes the peeing and pooping worse coz she is upset that half the house is now boxed off - though thats her own fault.
I have to be honest here, if that's your true view of a cat's behavior, it would be better for Mandy if you can find her a good home.
Not a shelter, mind you, but another, good home, where you can trust the people will take care of her.
It seems to me that you don't have the time, energy, and let's be honest, the patience and love to teach Mandy what's going on.
It would take a LOT more than treats for a few days for Mandy to stop peeing and pooping everywhere. Scolding especially doesn't help.
The fact that you shut Mandy out of your bedroom cold turkey where she has been allowed to be in all her life so far, made her turn to one of the only things available for a cat to let you know she's disappointed in you: peeing and pooping everywhere but the litterbox.
Hence, when you shut her out of all the other rooms you don't allow her to go into, the disappointment deepened, and the peeing/pooping out of place worsened.
It's very obvious. And it's not your cat's fault.
Maybe someone else will have a better advice for you.