If somehow the community thinks this situation is different and it warrants a refund, then as always we have no problem doing it as long as it doesn't open the door to other types of abuse later on.
I think this scenario is somewhat different if the user can indeed provide the correct letter of guarantee.
In the previous scenario, the only thing the user could provide was a signed message from the address he used to deposit. This is insufficient for Whirlwind to draw any meaningful conclusions.
In this scenario, the user can provide a letter of guarantee. Assuming this letter of guarantee is indeed the correct one, it will have a deposit address inside it. If the user can
also sign a message from the address(es) which sent funds to the address contained within the letter of guarantee, I would say that's pretty compelling evidence that the user is telling the truth and does indeed own those funds.
Happy to be corrected if I've misunderstood anything, though.