4.5 IdP initiated login
If you use a simpleSAMLphp IdP, and you want users to be able to bookmark the login page, you need to test IdP initiated login. To test IdP initiated login from a simpleSAMLphp IdP, you can access:
https://.../simplesaml/saml2/idp/SSOService.php?spentityid=<entity ID of your SP>&RelayState=<URL the user should be sent to after login>
Note that the RelayState parameter is only supported if the IdP runs version 1.5 of simpleSAMLphp. If it isn't supported by the IdP, you need to configure the RelayStateoption in the authentication source configuration.
As for account linking, it's my understanding that simple doesn't do this (it's getting out of the simple realm). To use it, you'll have to clean up accounts.
Actually, I suppose you could - though you'd have to build a structure to do it. You would need to somehow build a mapping of accounts from the corporate ID to the SP accounts at Wordpress, Google, etc.