Will she still go in the litter tray? If she doesn't that is the place to start working on a solution. Sometimes cats develop an aversion to their tray and then try to go somewhere else. Once they start somewhere you have to make sure it gets cleaned well and with non-ammonia cleaners to get the smell out. I would try some different kitty litters and give her an extra tray. See if you can move the tray to somewhere she would prefer going? Feliway spray lasts for ages and you can target it in the areas you want so if you go that way again I would use the spray and not the diffuser. If she has a real thing for the couch. I would put aluminium foil on it as cats don't seem to like walking on that, and put the tray next to it. See if she will go there as an alternative. Then you could move it in stages back to where you actually want it.