Welcome to the pets forum. The folks here are wonderful.
I have some experience with the splenectomy and following heart issues. In 2007 my 10 year-old Scottish terrier became gravely ill one night and required emergency spleen removal surgery. This was performed at an after-hours emergency vet center. My dog's spleen was engorged to the point of rupture. When we had the surgery, we were told that there was a potential for heart problems. The way it was explained was that removing the spleen caused the heart to "throw enzymes" (sorry I can't be more specific). My dog did have the heart complications and required staying several days in a vet ICU hospital so her heart could be monitored and she could have her medication adjusted. It was very touch-and-go for several days. When she was discharged she did have a few weeks of heart medication (don't remember the name) and I was told to get her back to the hospital if she collapsed. She also had to have additional heart monitoring (echo and EKG I believe) one week after stopping her meds. She was able to be taken off the heart medication and things appeared normal at her testing.
This is a very difficult surgery for a dog to recover from. Why was the spleen removed? Injury, cancer, unknown? In my Tipper's case, we were told it was probably cancer, but her biopsies were negative. For some reason she developed blood clot material in the spleen causing it to fail and engorge. We have no idea why it happened. With cancer, it is possible for the cancer to invade the heart or for small tumors to "bleed out" and cause blood loss, which in turn leads to weakness and pale gums.
One thing we experienced with our dog was that she developed lots of infections during her last year. The spleen serves as a giant lymph node and works with the immune system. So watch for infections.
I do understand how very difficult this is. My little dog wasn't very active and a true snuggle-bug lap dog so keeping her activity level down wasn't difficult. If it is at all possible, you may want to ask your vet for a referral to a vet cardiologist or a vet internal medicine specialist. If heart tests haven't been done, you may wish to do this. One of the big concerns was that Tipper could have had cancer in her spleen that had gone to her heart; she didn't. Recovery from this surgery was over 2 months for my little dog. Prayers for your sweet border collie