I think we all have moments when we think our dogs understand far more than they should. And it's a given that they'll always prove you wrong...whenever I tell someone how rotten my dogs are it seems like they behave like angels, and when I tell people how good they are...well, we won't go into that.
In the horse world it's become such a widely-held superstition that I know few trainers or riders who will brag about their horse before going into the show ring, for example!
In truth, though, we have to remember that our animals experience the world very differently than we do. I believe that they do understand more than a lot of people give them credit for...but I don't think that they're quite capable of that kind of thinking.
Instead, I think that there are other factors that we don't even notice that impact the animal's behavior.
In your example, I would wonder if maybe she was so attentive and well-behaved because she was engaged and excited by your visitors (even if they weren't actively playing with her), and then after they left she was tired or you just weren't as interesting so she ignored you. It's also possible that, since you haven't had help training her yet, you are not working with her in a way that gets her to listen. I see a lot of people using ineffective commands and body language that their dogs are prone to ignoring when it suits them.
I would also caution you that chewing out of the crate isn't necessarily separation anxiety. It can be, but it can also be a sign of a bored or understimulated dog. Unless you have other reason to be sure she has true separation anxiety, perhaps consider whether she's getting enough physical exercise and mental stimulation--1 year old GSDs are a handful!