RE: Portal - Direct link to SSO login on header
As per the Dynamics 365 roadmap this is possible - (roadmap.dynamics.com > Whats New). You will need to use the latest version of Dynamics 365 and Portals.
Support Azure AD-B2C for Portal authentication using a single sign-on (SSO) configuration -
For Portals that require a consumer based login, this feature will now support the ability to:
Configure your portal authentication to use a Single Sign-On configuration
Support Azure AD-B2C for customer authentication
Manage your Portal security in Azure
* depends on Azure AD-B2C availability in the region(s) Portals are deployed