Interesting read.
I have show birds and enjoy showing them once or twice a year. Even with quarantine it does not appear I could ever be sure I was MG free if adults do not always show signs.
There are several hundred exhibitors at these shows. The birds are all in close proximity to each other. I would bet my birds have it.
Because I have "pure" lines of birds, they lay less and have less vigor than crossbred birds do. Hybrid vigor helps... with my layers I a sure..
Show birds just do not lay the number of eggs a good cross does, so no way to gauge on egg production. I can see where it could be a problem, for you, specially with book learned non-chicken people that read -MG is bad kill all birds.
Your problem would be birds in other "allotments" and wind, tweety birds etc, will spread it back to your new flock is my fear.....