Edit: I typed this out earlier this morning and neglected to send it

. Sorry, some info has already been discussed.
This is on the federal level as of June 11th. A lot of good antibiotics are no longer going to available OTC. Terramycin and tylosin are on the list of exnayed drugs. Stores who have stock are currently permitted to sell whatever they have. This was my reasoning to have you get whatever you could quickly.
It is perceived generally as unethical to bring otherwise healthy birds into a flock with a life long, highly infectious disease. Ultimately, as long as your aren't getting rid of birds and infecting other flocks, bringing new birds in is a personal choice. Again, MG isn't fatal if managed properly, so it isn't necessarily a death sentence to new birds.
My mother has had an MG infected flock for over 10 years now. She hatches into her flock, but doesn't order any chicks in. MG can be passed to a chick through the egg, so it's assumed all the chicks she hatches are born with it. I was on the fence with this initially, but there is a an argument to be made about giving something life with MG, that otherwise wouldn't have had it.
And again, it is very manageable. She has hatched chicks that live a full 5 to 6 year life. Id say this is primarily why I now support her practice of hatching chicks into the flock.
She will spot treat the symptomatic birds, but doesn't do any kind of preventative treatments, or flock dosing. You'd have no idea her flock would have anything wrong with it just by walking though it.
For MG, yes. But I'm not certain to what extent it protects, or if it is like others where birds can still contract the RD, but just won't be symptomatic.