Pumpkin's Mama my heart is heavy for you. So many of us have been in this situation, and its never "the right time" for us humans to say goodbye to our four legged loves.
It doesn't matter how you procured Pumpkin. You saved her life. As you said, she could've been shipped away and disappeared without ever knowing love. She gave you 13 and a half years of unconditional love in return...you could say she's repaid the favor, even though you never asked her to. Your job now is to follow your heart and to listen to Pumpkin. She, and your heart, will tell you when.
I can especially relate to your sentiment about your pup saving you. I was in a very dark place when my Dante showed up. He was already 10 years old, and long story short: he needed a safe home emergently, so essentially he fell into my lap out of desperate need for his safety. I didn't think I could handle a dog in the state I was in, I was depressed, had recently been laid off, gotten out of an abusive relationship, and was in tatters, but my Dante gave me a reason for getting up. He needed me, so I needed him. Knowing that he was sick also made me look back at our short 5 years together and I hated that sometimes I'd been gone on vacation without him, how sometimes I just wanted to be away in my room and he'd be with my family in the living room, but I loved him very much, and tended to his every need.
When he was diagnosed with his heart murmur and enlarged heart, the vet said he should be better with medication, but my gut told me I had little time, even despite Dante looking to be on the mend. I'd spend my nights just watching him, sleeping in whatever position I could to make him comfortable and so I could watch him, and sure enough he was gone a week later, despite what the vet said, despite his outward appearance looking better.
I'm rambling and talking about my experience a lot but what I'm trying to get at is this: you know what is best for Pumpkin. She trusts you to see her signs, her tiredness, her pain, her lust for life gone, and to guide her in peace and dignity into the great beyond. Just listen to her, and understand that you are taking on her pain to let her be free, then you'll know when.
You're in my thoughts, and so is sweet Pumpkin.