1) In some cases, yes. If the WSDL contains things like Policies and such that direct the runtime behavior, then the WSDL may be required at runtime. Artifacts are not generated for policy related things and such. Also, in some obscure RPC/Literal cases, not all the namespaces that are needed are output in the generated code (per spec). Thus, the wsdl would be needed for them. Obscure cases though.
2) I thought something like would work. What version of CXF? That sounds like a bug. You can try an empty string in there (just spaces). Not sure if that works or not. That said, in your code, you can use the constructor that takes the WSDL URL and just pass null. The wsdl wouldn't be used.
3) Just the limitations above.