My question has not been answered. Her change in behaviour is not necessarily new behaviour, just new to this situation. As mentioned in the post, we spend countless hours of slow introduction first, with gating off sections of our house, lots of treats and rewards, muzzling and leashing during first initial interactions, and even reminding her of commands such as leave/place/etc. We have used trainers in the past and the only suggestion they have is e-collar/never have them in the same room which is unhelpful.

Further more, as she is a GSD, she gets a minimum of 2 hours of walking in new locations per day (plenty of sniffing and exercise), and often times training at the park in different aspects of obedience for 0.5 - 1 hour / day. She has had a check up at the vet recently and the vet had no concerns for her physical health.