Many a time it is very difficult to diagnose a primary cancer. From your question it can be said that she had metastatic disease to start with but what made you think mastectomy
and chemo may be a treatment option is not clear to me, anyway CHF and COPD
are relative contraindications for a major surgery even these can be tried in special circumstances. As you have mentioned your mom was on regular follow up at a hospital and lots of CTs , Bone scan Xrays Mammo were done, probably her breast cancer screening
was done regularly. there are many cancers which are not picked up by screening and with a very short period present with a metastatic disease. however, this is a depressing fact that your doctors failed to help you so much.