Ceejay (((hug))) :-(
This just wasn't an everyday dog on dog meeting was it hun, two grown men threatening you and both saying they
want to kill your dog! Sorry, but that just isn't a normal reaction is it? More of two bullies picking on a lone girl/woman totally out of order and uncalled for.

Never mind the dog, but both men need reporting to the police for threatening such a thing. (It may even be worth contemplating if you seriously feel for yours or your dogs safety.)
The other man knowing the history of his own dog and even telling you "It's ok, he has a ball in his mouth" just goes to show how foolish and unknowledgeable he is of even his own dog, a ball in the mouth will certainly distract his own dog from growling, but in no way will stop it from reacting to a growl from yours, if a Collie is that way inclined, it will react no doubt about it, he is blaming you for his dog becoming aggressive and to be honest his dog may well be great with other dogs who show no 'challenge' but knowing yours growls he should have popped his on lead, I expect his aggression towards you was down to his own misjudgement of his dog. (Foolish pride) The point is his dog was off lead yours was on and his attacked, so the blame this time was purely at his door.
From what you have always said she growls and chases but has never actually attacked or bitten another dog, but as I've said previously BC's often communicate in this way, if I met your girl every day, I'd probably just walk past and say "Shut up Meg!" which seemed to work with other Collies I got to know. :-D But then it got them used to looking at me for a pat instead of the dogs, I know you can't do that with strangers but that is how quickly regular walkers can stop her behaviour.
If I didn't know Collies I may well perceive your dog to be looking for a fight or class it as unfriendly if she growled everytime I passed her, there is nothing we can do about that, other people and other dogs will see her that way ceejay. As the man you met has done, pop a ball in her mouth to stop the growling, continue with your everyday training and continue to lead her when passing dogs you don't know or who are likely to go for her.
You were not in the wrong at all this time ceejay, hold your head up, remember if worried contact the police, these men were being nothing more than bullies, that dog should have been on a lead whilst passing yours, keep working with Meg, hopefully and eventually through training you will be able to stop her growling to a command of Stop, Shush etc, but a ball certainly will be a great deterant for now.