I don't understand why they can't restore from a backup. Sure, it may restore things that no longer were in the bank when it was lost, but it's Blizzard's screw up.
It's the guild bank, so it can't hold soulbound items etc that could actually have mechanical issues with dupes.
Office buildings are not built for housing, it's often very expensive to convert them.
(More plumbing requirements, legal requirements for natural light etc)