Usually, a mass isn't removed, but the whole spleen is removed and then is sent to a pathologist to determine whether the cancer is benign or malignant. Regardless, without taking the spleen out, there is a risk of splenic bleeding which can cause death even if the mass is benign. Therefore, the mass on her spleen could likely be the most pressing problem at this time.
Generally, splenectomy can be done quite safely in dogs with early CHF. However, every situation is different, and the risks may be higher if the dog is very old or frail and the surgery may not be a good option if there are signs of cancer spread to other organs.
Consulting with an oncologist may be helpful for expert advice. The oncologist may suggest having a CT scan done to see whether there may be signs of cancer spread to make a more informed decision based on the findings.
If the oncologist feels surgery is warranted, it would be best to have it done by a board-certified surgeon and in a place where the dog is monitored for a couple of nights in case there are complications such as arrhythmias.
Nowadays, there are sophisticated new anesthetics that are fairly safe for dogs with CHF; however, there are always risks with surgeries and anesthesia and the recovery time. Palliative care is still an option when things are too advanced, and the chances are too great. So these are all things you want to talk about when consulting with your vet.