I'm sorry that I was off my computer when you posted your additional questions about Amber.
I know you feel awful about this and my heart goes out to you.
I am sad to say that your research is accurate: about 90% of mammary masses in cats tend to be malignant. But it's possible that Amber will be in that lucky 10%....there's no way of knowing at this point.
Removal of the entire chain has been shown to improve survival as long as there hasn't been metastasis. In fact, some vets recommend removal of both chains which sounds pretty drastic, I know, especially for a cat her age.
If it turns out to be cancer, then there are other options for her such as chemotherapy which would be best pursued by consultation with an oncologist after biopsy results return. The kind of tumor and staging (basically how malignant the tumor might be and if it's spread) will determine what, if any, additional treatment options you might want to consider.
You may find this article of interest in terms of survival times:
Feline mammary adenocarcinoma: tumor size as a prognostic indicator Can Vet J. January 2002;43(1):33-7. Jodi R Viste1; Sherry L Myers; Baljit Singh; Elemir Simko 1Department of Veterinary Pathology, Western College of Veterinary Medicine, University of Saskatchewan,XXXXX Saskatoon, Saskatchewan S7N 5B4.
Mammary carcinomas and adenocarcinomas (MACs) are relatively common tumors in cats. The postexcisional survival period of affected cats is inversely proportional to tumor size, but the reported median survival periods for different tumor size categories is quite variable. This variability diminishes the prognostic value of reported data. In our study, cats with MACs greater than 3 cm in diameter had a 12-month median survival period, whereas those with MACs less than 3 cm in diameter had a 21-month survival period. Survival periods for cats with MACs smaller than 3 cm ranged from 3 to 54 months; therefore, tumor size alone is of limited prognostic value in cats with MACs smaller than 3 cm in diameter. In cats with MACs larger than 3 cm in diameter, tumor size appears to have much higher prognostic relevance, because this study, as well as others, have indicated that cats with MACs greater than 3 cm in diameter have a poor prognosis, with median survival periods ranging from 4 to 12 months.