Use case notes

1.   Use case Major-minor

The first problem is addressing "The system identifies that it is a major/minor version change from B" wrt the instance in scenarios A and B. I'm going to assume a version of the UBL solution, and say that this is accomplished by requiring new elements which are semantics-changing == 'must understand' to be in a different namespace from the target namespace of the B schema. Detection can either be done OOB, or by changing the wildcard in B to be namespace="##this", in which case the W(B) will not be schema-valid per B.

This solves all of the four sub-cases as written. Here are the schemas in detail:

<xs:schema targetNamespace="B">
      <xs:complexType>
       <xs:sequence>
        <xs:element name="title"/>
        <xs:element name="author"/>
        <xs:element name="price"/>
        <xs:any minOccurs="0" namespace="##targetNamespace"/>
       </xs:sequence>
      </xs:complexType>
     </xs:schema>

     <xs:schema targetNamespace="B">
      <xs:complexType>
        <xs:sequence>
         <xs:element name="title"/>
         <xs:element name="author"/>
         <xs:element name="price"/>
         <xs:element name="editor"/>
        </xs:sequence>
      </xs:complexType>
     </xs:schema>

    <xs:schema targetNamespace="W(B)">
     <xs:import namespace="B"/>
      <xs:complexType>
       <xs:sequence>
        <xs:element ref="B:title"/>        
        <xs:element ref="B:author"/>
        <xs:element ref="B:price"/>
        <xs:element name="discount"/>
       </xs:sequence>
      </xs:complexType>
     </xs:schema>

What about the larger issue raised, namely "This use case [should extend] to multiple levels of versioning". If we assume that changes are monotonic, we can add another <xs:any minOccurs="0" namespace="##targetNamespace"/> and it will all work OK.