If 2. is implemented using fenced code blocks with attributes, that makes sense. Otherwise, the implementation could always keep the raw strings contents of the container in the AST (in addition to the parsed markdown-content) so that plugins etc. can do with it whatever they want.
No, the container blocks are closed with ::: on its own line, while leaf blocks are not. For nesting, a different number of colons can be used, using the same rule as currently for fenced code blocks.