Having these spleen masses is not uncommon in these aging labradors. These masses can be benign, but they can also be an aggressive cancer. The risk that exists even with benign masses is that the mass can rupture and they can bleed to death into the abdomen. They can also cause arrythmias in the heart. This can make surgery risky, but a preoperative EKG can rule out an arrythmia. In the end, I would recommend removing the spleen and tumor and having it submitted for biopsy. If this is a benign tumor things should go well. IF this is malignant then our long term health will be in question. The good news is that the ultasound showed no other masses in the abdomen. This would indicate a high probability of spread. Also, during surgery, your vet will be able to tell if there is any obvious signs of spread throughout the abdomen.
I do recommend surgery in these cases because if you try this surgery if the mass is actively bleeding the prognosis is poor. Also, to get a diagnosis without surgery would mena a ultrasound guided biopsy or cytology which could rupture the mass. I know this approach is aggressive, but a conservative approach to spleen tumors has not gone well for me in the past. So, before surgery, I would make sure that bloodwork is all normal including a coagulation panel, and general profiles (and CBC sent to a lab as you get human eyes looking at the blood instead of a computer like the inclinic blood machines), along with the EKG. At this stage you do not want to put her through a major surgery if this has spread (and we can find it). This can get expensive, but this preliminary workup gives us the best chance for a positive result. If she is losing weight or muscle tone, or she coughs up blood, I would be more concern for a situation that this mass has spread.
I know this is a lot to take in, but this can be a complex problem and you may be making a decision without a lot of information. Many times, we don't know how bad or good it is until you get into the surgery and get to see what is going on the abdomen.
I hope this helps and gives you direction
Feel Free to follow up, I know this is alot.