Well I guess I understand wanting to lock in a good rate, however most of the time a lock period is only 30 days. I am sorry to hear about what happened. Unfortunately there are alot of variables involved that buyers need to consider with short sales.
For instance, what if the lender would have accepted the short sale but not released the note? In orher words the seller would have still been under an obligation to pay off the note even though it would have not been secured by the property. Similarly what assurances did you have that the full WRITTEN approval letter would have been delivered in a quick enough timeframe for you to close in mid April (beyond of course the constant verbal reassurances from the listing agent)?
Again, I am sorry to hear about what happened, however with short sales, investing any money or making any plans prior to having that approval letter is risky plain and simple. It does suck though.