I am very sorry to hear how this turned out. I really thought with how quickly the swelling would come and go, that it had to be air under the skin. Did your vet give any insight into how the tumor caused such drastic changes in swelling? Bleeding internally? Did you submit the tumor for histopathology to find out what kind of tumor it is? It is unusual to see cancer in such a young snake. Although now that I say that, the only other heart based tumor I have ever personally seen was in a 4 year old kingsnake.
One last thought... you may want to reconsider keeping and breeding offspring from this snake. I recognize that he was a beloved animal and it is tempting to keep that memory alive via his bloodline. However he also had a very strange cancer and there is no way to know if his offspring might be predisposed to a similar condition.