Awww, I'm sorry, Uncle David. I had a cat that, due to very strange circumstances, had to be taken back to the shelter, and it was very tough. All the best, and I'm glad that Greta is sticking around.
Also, might I suggest you look around with a black light for any leftover urine marks? Everything will show up that way, and if you get them "neutralized," that should help a lot with Greta.