YESSSSSSS
it can be either one. “contents” can be used to describe items in a location.
it can’t. contents is a countable noun, hence ‘table of contents’ (describing a list of content). in this use case, there is one piece of content.
no, there are multiple contents (words, images, links) that you can enter in the post box
imagine the post box was the table of contents and the words and elements are the contents