Okay, I have some breaking news!
After much pestering of customer support to try to get in touch with someone who is able to share information pertaining to this issue, I have been able to obtain the answer to many people’s questions here.
Hopefully, I won’t get in trouble for sharing ticket info, but in this circumstance, I think it is really necessary and vital for people to know, so I am going to take the risk of posting this. According to what I was told, “Yes, the guild bank bug is fixed, and it is safe to use the guild bank”, and “Yes, players are having their items restored, it is just taking longer than expected”.
imgur dot com/a/Atkwfyy
So, if what I am being told is accurate, the bug is fixed, and items should not be disappearing anymore. The guild bank is safe to use to deposit items. Furthermore, they ARE restoring every lost item, it is just going to take a long time; longer than they expected. This is honestly no surprise, considering how many people and their banks were affected, which constitutes a massive number of items needing to be restored, which no doubt would be leading to a complex and time-consuming resolution. I don’t know why Blizzard could not officially put this information out, but at least now we have some piece of mind and hope that this problem will be righted in the future.
I also want to thank the customer service representatives who were patient enough to deal with me and answer some of these questions. 
Some of them are good guys, but in a difficult position of being told to keep their hands in their pockets about certain issues. Their job isn’t easy, and I think they deserve a bit more empathy than they get about certain issues. Try to keep that in mind, when you are frustrated with something.
In any case, if I hear anything else about this issue, I shall keep people updated.