My heart goes out to you Maggies mum.
We are in a similar situation , my mum's yorkie Pebbles was 15 and doing well never a days illness and suddenly in january she started not eating not drinking...
We took her to the vet and they said she had gone into liver failure , it was so fast we are heartbroken
we had to make the dreadful decision to put her to sleep but in all honesty I dont think she would have made it through the night anyway and we wanted her to be free of any pain.
You can go 2nd guessing yourself , you did the best for her at the time and she knows she was loved and led a happy life,that is as much as you can hope for any little one but to get 15 years is a great feat both for Maggie and Pebbles , RIP little ones.
No one will ever replace them and they will always live on in our hearts.