1. Depends on the new project, of course. Just like any language.

2. Same thing. You can construct an example that breaks any language in existence.

3. Yes.

4. Yes.

5. Yes.

6. You’re using an idiosyncratic definition, so yes, it’s not the same. Backwards compatibility is about the same thing continuing to compile, not about changing things and expecting it to still compile; that’s forwards compatibility.