I wont give opinions on legality's, but I have been given a couple of pieces of advice over the years that are on point.
When you have this long story on why your particular configuration is legal..ask yourself..how will it sound in court?
To 12 people who have probably never shot anything.
The other was given to me by a atf agent. Get it in writing. They do this all the time as a course of business. Remember just because they gave you the nod over the phone means nothing. They are just slobs going to a job like the rest of us.