We had our second training session at the sanctuary, so the trainer could see how Celeste is with other dogs.
Turns out I made the issue bigger than reality.
Celeste's initial greeting is appropriate. She's relaxed but prefers nose-to-nose, which I've never thought was appropriate or polite. She can walk side by side with other dogs and generally ignores them, even if they are off leash and she is on.
The trainer and I agree that she then becomes unsure of what her role in the new relationship is, she tenses and then will either growl or lip curl. She was inappropriate with a smaller dog, but responded well to the dog's appeasement behavior (on her back, eye avoidance).
In fact, the trainer thinks the greeting behavior can be modified in a month, if not less. She thinks group classes would be a great idea, so I'll look into those. After she gets good at nose-nose, we'll work on nose-butt greetings and eventually work our way up to off-leash play time. Today gave me great hope that Celeste will be able to play appropriately with other dogs.
The trainer commented I was really good at keying in on Celeste's behavior, knowing the right moment when to call her over and reinforce desired behaviors while distracting her from undesired ones.
I can thank Mina for that. I can also thank Mina for my rather over-the-top belief that Celeste was irremediably damaged and would never ever play nice with other dogs off leash. Mina is a difficult dog, and she is also a more subtle dog to read. I'd always considered myself capable of understanding most dogs' behavior, but Mina proved me so wrong. She taught me much about appropriate and inappropriate behavior, about the less obvious cues and insight into a dog's inner state (or our best approximation).
Relax! That is what I'm telling myself now. We'll work on the dog greetings with Celeste, at work and while on walks. I'll remind myself that Celeste is actually acting quite normal and isn't behaving as inappropriately as I thought. And hopefully, Celeste will make some canine playmates other than Mina. It would be so fun to go on weekend hikes with friends and their dogs without worrying about a grumpy Celeste (Mina does not do long hikes, thanks).