Dear Rosie's mom, yes, you are correct, you do not want to "correct" a dog for growling, you have heard right. That's great you know this!!! yay!!!
This won't make a dog "Nice"
only silent.
The dog learns to never growl, but his inner attitude is just the same. Then you are left with a dog who never growls, and is very difficult to read, especially for strangers.
The owners of dogs who have been corrected for growling report things like "We just never know when Fido is going to bite, one minute he is fine, the next minute, he has bitten someone!". poor Fido, he's been trained to not give a warning prior to a bite. sad.
not that your dog would necessarily escalate to a bite, i'm just saying, you are spot on about do not teach a dog to never growl. Just remove dog away from whatever he is growling about.
I think Rdog has given you some great advice, i hope it goes well. Re Rosie isn't liking the muzzle, might be too late for this tip, but, maybe if you start with only brief times of wearing harness, reward heavily,
i do mean heavily, using real meat, liver, cheese, peanut butter, lavish praise, etc etc.
and then,
keep repeating it, keeping harness on just a bit longer each time.
I understand about your tears, Rosie Mom, i do, i'd feel that way too.
It's never too late to start all over when it comes to dogs,
maybe,
you could get NEW, completely different muzzle, and start all over, closely following the video above, and keeping muzzle on dog only super briefly, making that moment the best part of Rosie's day, and repeat this moment a few times each day,
while slowly, slowly, slowly increasing time Rosie wears it...not sure, but, maybe worth a try.
I've seen dogs who loathed leashes, get over it, when a new method, on a new leash, was used. ya never know, *might* be worth a try. HUGS!!
HANG IN THERE AND BEST OF LUCK!!!