I'm so sorry to read your little dog is so ill. Hard to tell what it might be. The electrolyte imbalance can be caused by the diarrhea. You mentioned they were considering surgery? On what? Sounds like they didn't see anything obvious. I assume they did blood work? The results should be some indication if it is pancreatitis. What is normally done is all food is pulled for at least 24 hrs and fluid therapy and possibly some meds are given. Do you happen to know why they thought he needed a blood transfusion? Sorry, this could even possibly be Cushings or Addison's but blood work should indicate something in that direction. Often, Vets think there is a kidney problem and further testing and symptoms prove to be Addison's. Wish I could be more help but will be keeping fingers crossed the Vet can figure this out, it is manageable and your little one will be home soon.
OK try not to panic. I understand, I love my dog too and when she was in the vet's for her emergency surgery I was like a mother hen.
Pancreatitis is very unpleasant, and he will have to be fasted for a while, and will most likely receive fluids by IV. Electrolytes being low is one of the things that happens with severe dehydration. Bad diarrhea and/or vomiting can cause severe dehydration very quickly. I am sure the IV will stabilize him soon.
He could have problems with his liver, that can cause a swollen stomach. Fluid retention in the abdomen (ascites) can cause that too. That can be caused by a number of things. Has the vet checked his heart?
Attacks of pancretaitis can be horrid, but most often pass. He will be on a strict no-fat diet afterwards, with absolutely no Human food at all. That may help prevent any further attacks.
Let's hope that the treatment he is receiving will help him to fight this, and regain his strength gradually.
But I do understand the agony you are going through. I would be the same if it were my girl.