How can I be sure that my information will not be shared without my knowledge?

You, and not the platform, decide which information is shared with whom. You may even determine for each document whether it may be shared or not, in order to avoid unpleasant situations. You also decide when you wish to stop the sharing of documents.

After your death, depending on your decision as to what will become of them, the contents of your vault will be destroyed, either directly (option 2&3), or after having been transferred to heirs (option 1) (see question What becomes of my vault and its contents after my death?)