<Call me an optomist, but Susan didn't die of something as dumb as licking
200 envelopes. The doctor that pronounced her dead probably made a mistake and she never died. She was presumed dead, but got better and came back to life.>
Susan is my favorite charater on television and thus on Seinfeld, but not even I can bring myself to believe this. Seinfled plot lines don't continue from one season to the next. This is the last we'll hear of her and her death. That's what makes the entire situation so unbearable since, as you say, it was too "dumb" to be even remotely believable. If she is brought back, it would be the equivalent of Joan of Arc leading the French to victory in the 100 Years War against England (who at the point of her intervention controlled most of France); that is a full fledged miracle.
"Kill all the Prussians and we'll have peace." A Saxon-German Soldier, 1914