I too was diagnosed Stage IV like your mom. I had 7 lesions in my liver (all lobes were affected) and a tumor in each lung. They gave me a 6-month life expectancy without chemo, and 18-24 months if I managed to "do well" on chemo.
After 13 months of chemo and considerable shrinkage of all tumors, my local oncologist still said I was inoperable due to the number of tumors. I got a second opinion at Memorial Sloan Kettering Cancer Center in NYC with Dr. Nancy Kemeny. She said I was a good candidate for surgery!
Six weeks later I was underwent a liver resection and had an HAI pump implanted. The HAI pump delivers chemo directly to the liver. Three months later I had double-lung surgery to remove the lung tumors. That was over 5 years ago. I am completely free of all cancer now.
I know exactly how scared your mom is because I have been in her place. Don't give up! There are many doctors who discourage Stage IVs from anything but palliative treatment, but I guarantee you there are others who will fight for her life. If at all possible get an appointment with Dr. Kemeny. If NY is too far away for you, ask MD Anderson about getting an HAI pump. I believe they also have an HAI pump program there, although Dr. Kemeny is the expert in the field.
Your mom's drop in CEA is a very good sign
that the chemo is working. Her CT scans should be showing a reduction in the size of the tumors too. Now is the time to find someone who will be aggressive with her treatment. It doesn't sound like her docs are mentioning all the options out there -- HAI pump, immunotherapy, TIL, etc. Those things should have been discussed with your mom. It leaves me thinking she needs a doc who will be more aggressive on her behalf. Stay strong. Good things are still possible and I am the living example of that.
Let me know if you or your mom have any questions about the HAI pump.
Below is a link to Dr. Kemeny. https://www.mskcc.org/cancer-care/doctors/nancy-kemeny
And here is a link to a video about our Sleen and her TIL therapy.https://www.youtube.com/watch?v=Talj5pIrMLw
Hope this helps!