I am sorry to hear you are worried about Chippy. My name is Rebecca. I am a veterinarian and will do my best to help.
If there is a mass in her lungs, that could be from the malignant mammary tumors spreading to the lung. That sometimes happens with these mammary tumors. Any chance you can get the Xrays and post them here?
We normally do not do surgery for lung tumors, unless we are sure it is a solitary tumor. Metastatic tumors, ones that are spreading to the lungs from other areas like mammary tumors are rarely solitary, and surgery is not the treatment. These are usually treated with chemo or radiation therapy. A tumor or mass on the heart is even more of a concern. These are rarely operated on because of the risks for bleeding.
If she were my own dog, I would probably do the ultrasound to get a good look at the heart and the internal organs. If tumors are seen, it is possible that a needle biopsy, using the ultrasound to guide the needle, will get a diagnosis of whether this is cancer, what type of cancer, if it is metastatic from the mammary tumors or not. Ultrasound is not invasive, and she could be sedated for the procedure.
Once you get a biopsy, get a diagnosis, you can talk to a veterinary oncologist and find out your options: surgery, radiation, chemo. You can find out the prognosis with treatment, and without.
If she had 5 malignant mammary tumors, and now there is evidence of tumors on her kidneys, lungs and heart, this does not sound good. It sounds like she may have cancer spreading throughout her body. If you can find a veterinary oncologist who can communicate with you, it might be best to start there, with a consultation, before doing any more tests.
Let me know how she is doing and what other questions I can answer. I am so sorry you have gotten this bad news about Chippy. Rebecca