That third picture is sooo adorable, she looks so happy and content.
I can only reiterate what others say, you will know when the time is right to give her the final gift of love and let her go.
Having lost my big lad John recently to bone cancer recently, I bought him a harness to help support and take the strain off his legs. I could hold the handle at the top and help him toddle around, which may be something that may help Ruby in the meantime -- or even a towel or sheet under her belly, which you can hold to help her and make her feel safe.
