Hi, my area is more lymphoma than leukemia. But I can give my opinion that although "acute" leukemias are the ones that progress quickly, I don't believe they typically go from undiagnosed to fatal that fast. With cancer, almost anything is possible but there still is the matter of what is very likely or very unlikely.
Yes, the very common CBC test would show leukemia.
There would likely be easy bruising and bleeding. While other end stage symptoms such as fatigue can be caused by being extremely overweight.