Does the FHA restriction on dual employment apply to all employees of an FHA Mortgagee?

No.  The dual employment requirement applies to Mortgagee employees who are involved in activities related to FHA Mortgages.
HQ Policy Determination

All policy information contained in this knowledge base article is based upon the referenced HUD policy document. Any lending or insuring decisions should adhere to the specific information contained in that underlying policy document.

Topic Number: KA-02876