No it would not be sufficient for anything and to go through all that hassle just because someone using their service has made a series of error is not productive nor a good signal to send out to others who use the service incorrectly.

Having said the letter of guarantee is sufficient and the address within it should be the address the rightful owner is capable of signing a message. If the owner cannot sign a message from the address because it was going to a coinjoin address, the owner should not have his mistakes paid for by Whirlwind. What would happen if Whirlwind paid him for his mistake and then a few days later another member presented the same letter of guarantee along with a signed address claiming he is the rightful owner?

If the circumstances surrounding stormbounty losing his private key are correct, he will have to both take and accept the loss and then move forward. If he wants to use Whirlwind again he will have to keep in mind how it works otherwise he should use the services of an alternative he finds more convenient.