This is about the representation and alignment of the pointer object, not about the object being pointed to. And C requires struct pointer types to all have the same representation and alignment. This is generally necessary due to the possibility of having pointers to opaque struct declarations in a translation unit.

Regarding your second point, if I understand the model correctly, there is only an ambiguity in pointer provenance if the adjacent objects are independent "storage instances", i.e. separately malloc'ed objects or separate variables on the stack — not between fields of the same struct.