Confirmation Required - Hybrid Join, Entra Connect Sync and Different AD/Entra Usernames

Dear Microsoft Team,

We require your assistance in validating the following customer requirement.

The organization does not want users to switch their Windows profiles from the Local AD account to the Microsoft Entra ID account. The devices should remain AD-joined, and users should continue logging into Windows using their existing Local AD credentials.

The current environment has different usernames:

• Local AD user: 234ganesh@iss.local
• Microsoft Entra ID user: ******@iss.com

We are planning to use Microsoft Entra Connect Sync and Microsoft Entra Hybrid Join.

Could you please confirm the following:

1. Can the Local AD user and existing Microsoft Entra user be correctly synchronized/matched even though their usernames/UPNs are different?
2. Can the device remain AD-joined/Hybrid Entra joined while the user continues to log in using the existing Local AD profile?
3. Will Intune, Microsoft Defender and other Microsoft security policies continue to apply correctly without requiring the user to switch to an Entra ID Windows profile?
4. What is the recommended identity-matching method for this scenario — UPN/SMTP matching or sourceAnchor/ImmutableId (hard matching)?
5. If the Local AD user and Microsoft Entra user are successfully matched as the same identity, do we need to assign separate Microsoft 365/Entra licenses to both accounts, or is one licensed Entra identity sufficient for the user?
6. Please also confirm whether this approach has any impact on the user's existing Windows profile, mailbox, OneDrive, application access or Intune enrollment.

The main concern is that the Local AD username and Microsoft Entra username are different, and we want to ensure that synchronization does not create duplicate cloud identities or impact the existing user profiles.

Please confirm the Microsoft-recommended architecture and any prerequisites or limitations for this scenario.