Please find my answers..
a) Yes.. WSO2 Identity Server supports SCIM which is the standard approach for identity provisioning. Also there are web service APIs, that expose user management functions. You can front those web service API using WSO2 APIM and can expose as REST API.
b) Yes.. Apart from REST and web service API, Identity Server supports for authentication methods such as SAML2SSO, OpenID, IWA and so on.
c) Yes.. same as a) you have both options SCIM or Web service API fronted with APIM
d) There is an authentication framework in the WSO2IS, that users can be authenticated with different server.. Basically WSO2IS can act as federated IDP that application knows only to talk to WSO2IS.. WSO2IS can authenticate users from any other IDP, It can be drupal as well
e) Yes.. XACML is supported by WSO2IS... Its PDP API has been exposed via web service API.. but not as a REST API.. but as i mentioned earlier, you can expose it as REST using APIM. However REST support for XACML would be available with the further release