1) It looks like it is possible to update the User ID. There is a feedback item in User Voice for this that was updated.
Here are the steps:
1) Update the permissions for Workday Integration System User to allow username writeback. Add Get and Put operation support for “Workday Accounts – Domain Security Policy”
2) Modify the Workday Writeback attribute mapping to map Azure AD userPrincipalName to Workday UserID attribute
3) Clear current state and run full sync
Business phone also appears to be possible and the steps for adding that are documented here: https://video2.skills-academy.com/en-us/azure/active-directory/saas-apps/sap-successfactors-writeback-tutorial
2) The document says, "The Azure AD Provisioning Service runs scheduled synchronizations of identities from SuccessFactors EC and identifies changes that need to be processed for sync with on-premises Active Directory."
Based on this it should identify any changes and not just the ones you mentioned. Where do you see that it only triggers for name, title, or manager field?