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At this age, I would not use any valium without pre-use bloodwork performed by a vet.
Valium usage in cats can be done, but usually not for the reason that you are describing and not without a prescription. The dosage for a cat is quite low and a normal human valium will not work.
Valium depresses the central nervous system in cats and that is part of why it should be avoided in cats this age unless blood work and exams prove that she is completely well.
If she is peeing and pooping outside the box, there are things that you can do about that without using valium.
There is a great product called Feliway that you can purchase. You can use this where she has inappropriately soiled and she will not use that spot again.
I hope this help some!
Let me know if you have more specific questions. I would be happy to help more!