Personally, I never really liked the info string without attribute braces. As such, I’m in favour of (2).
However, I see the problem of backwards compatibility, but it shouldn’t be too disruptive if an info string of a single word is still allowed, i.e. the really bad case is:
``` hello world
this is NOT inline code with one backtick ` and two backticks ``;
it is a code block!
```