I agree with you on the one flaw with my analogy of there being only "one" you. That's a fair point. However, I think the housing analogy is even less appropriate. If you sell a house, whether it's your only home or if you're a developer who has multiple homes, you can show any given home to as many buyers as possible before accepting offers. And even then, people actually bid for your home. And people only buy homes relatively infrequently, so word of mouth is relatively less important than in the case of editors buying stories. If I were to apply the housing analogy to short fiction markets, I would be able to submit a single story to as many different editors as possible, and if there were any indication of interest, it would initiate a bidding process. I'm not arguing that there should be a bidding process, just that one would likely exist if the short story market were a direct substitution for home-buying. Oh, and you'd have to work through an agent in most cases to sell your stories. Actually, the housing market turns out to be a direct analogy to the novel market!
In regards to the probability of selling a story, there are also certain things that greatly enhance your probability of selling a story. The one most indicative of that is already having sold a story to a particular market. In fact, Trevor has admitted that he looks at the work of published authors much earlier in his process than unknowns. I'm not arguing it's right or wrong. In fact, there's a great commercial reason for it - big names attract readers. It is almost certainly the case that a poorly written Stephen King short story will beat an excellent written unknown writer's short story every time - and for good economic reasons.
And that's the reason I argued that it made far less sense for a professional author to sim-sub than an unknown. And in the case of an unknown, the only data you have are the acceptance rates on Duotrope to go on, so for the average given author, those are the most appropriate probabilities to use -- after all, it's the only concrete data one has. Sure there are half a dozen reasons why one story might get rejected, but as an unknown one has no data to assess that, so he or she should use the averages.
But I think while we may disagree on the relative risk of sim-subbing (I think the probability that two editors decide to purchase one's story simultaneously is over 900 times less likely than my dying of an accident this year, you, I think, believe it is somewhat higher), I think we both agree that the reputational consequences of a dual-sale could be extremely dire. Each writer should decide what level of risk he or she is willing to assume and run with it.
I'm curious as to the rewrite suggestions. Once you fixed X, did you mail it directly to him or put it back in the slush?