But what if I want to name the inner struct so I can refer to Outer::Inner (for example, for use with sizeof or similar, or to create a temporary to hold a copy of that type) later? Do I need to use typeof(Outer::inner)? And then of course multiple members of the same type...

You can't name the inner struct. So here are the options:

Say that you want this from C:

    struct Foo
    {
      struct Bar {
        int x;
        int y;
      } bar;
    };
    struct Foo f;
    f.bar = (struct Bar) { .x = 123 };

The struct in C3:

    struct Foo
    {
      struct bar {
        int x;
        int y;
      }
    };
    Foo f;

It is correct that we don't get the inner type, but because C3 has inference for {} assignment can happen despite that:

    f.bar = { .x = 123 }; // Valid in C3

You can grab the type using typeof if you want, but interestingly there's also the possibility to use structural casts, which are valid in C3:

    struct HelperStruct
    {
      int x;
      int y;
    }
    HelperStruct test = (HelperStruct)f.bar; // structural cast

But the normal way would be to create a named struct outside if you really need a named struct, e.g.:

    struct Bar { int x, y; }
    struct Foo
    {
      Bar bar;
    };

That ... actually seems reasonable?

If you need it to be first class visible outside, you declare it outside.

I believe I could live with this.

Also, just to be clear, the removal of semicolons deliberately makes the language whitespace sensitive, right? That is,

    struct Foo {
    } foo

declares a variable of a new empty struct type, but

    struct Foo {}
    foo

is a syntax error?

C3 doesn't have declaration of structs/unions in variable declarations. So both are equivalent and are syntax errors.