The Airmen Certification Branch, AFS-760 must have the Civil Aviation Authority (CAA) verify the validity and currency of the foreign license and medical certificate or endorsement before you apply for an FAA certificate or authorization. The processing of the Verification of Authenticity of Foreign License, Rating, and Medical Certification form takes approximately 45 to 90 days to complete. NOTE: Do not schedule any travel and/or checkrides, etc until a valid Verification Letter has been obtained from AFS-760.
This is NOT required for a standalone licence this is only needed if you want an FAA certificate based on a foreign licence which would absolve you from taking any of the tests but is much less useful.
The issue of the part 91 only issue can be resolved with a part 135 check at any recurrent assuming that opportunity comes your way.... Very unlikely unless you become a US citizen.