Be aware that your dog, now a teenager in dog terms and very nearly adult, is a genetically nervous and fearful dog. Some you can help. Other things it is better to manage and not try to change.
She should not be required to be friends with every dog she meets. Just like people, dogs have opinions about other dogs. As she matures she may be less inclined to like all dogs but may be selective. The situation you describe sounds like a dog park. I would stop going to the dog park. Instead, see if you can find a few friends with dogs and have play dates with just those dogs. Keep an eye on Luna. If she starts to get snarky, take her out of the group. Hackles up and ears down is a dog that is unsure and afraid. Hackles up to make her look bigger and ears down to appease. Learn dog behavior signals. She is telling you she is uncomfortable. Advocate for your dog and remove her from situations where she is uncomfortable. Again.. trusted friends a few dogs, the same dogs and the same people will help a lot. Liking all dogs is a puppy thing. She is now growing up. Adult dogs do not always like other dogs. Some do not like any other dogs.
In my world I do not let my dog play with other dogs. I have two dogs and I rarely let them play together (I compete and cannot risk injury). Dogs do find with NO dog friends.
Put Luna up when people come over. Do not have them interact with this dog. When she comes over to them I bet they are standing up. They bend over to pet her and that is VERY threatening to a dog. She does not need to be friends with your friends. After they have come in and settled down IF the people will actually do as you tell them and IGNORE THE DOG.. NO petting, NO eye contact, NO talking to her then MAYBE let her in to be part of the group. She is a fearful dog and it pushed she may bite and that will make her a huge liability (or a dead dog).
This is a very fearful dog. This dog's next move may be to bite. Manage the situation. This is what crates are for. This is why you need a place she can be put that is quiet and comfortable away from your guests.
I have one friendly and confident dog and one that is nervy and less confident. They are German Shepherds. I value them greatly. When I have company they are put up. If I have over night guests that can be a bit different but I am selective. If a person so much as SUGGESTS a dog of my breed has touched them aggressively I could end up with a law suite or lose my dog. I manage the situation by separating my dogs from my guests. I have kennels and crates for this purpose.
Just keep sending her to her bed and give her the treat there. Try to get her to default to her bed when she is worried. This is why I like a crate. It is like a den and gives a dog a secure place to go if they need a time out. Dogs that use crates often choose to with the door open. It feels safe.
Feeding her treats when she is reactive to the stuff outside may back fire. You are showing concern and then feeding her.. think about that a minute. It is unclear on your part and she may actually associate the barking with the reward.
She is NOT being protective. She is showing fear. She views you as incompetent to take care of the things she worries about. She thinks she must take care of things herself. You already KNOW she is fearful but advocate for your dog. Get between her and the thing that worries her. You face it. Put her behind you. Let her know it is not HER job and YOU will take care of it. She knows you won't advocate for her at the dog park (take her out when she is worried) or with company (put her some place quiet and safe when she feels insecure) so she has no security that you will advocate for her out on walks.
She is showing fear and is over threshold. Dogs are associative creatures. She is fearful AND associative so when she is over threshold, she stays there and associates that with the place.
It might be a good thing to simply take her to the same place all the time for her walks. Honestly? A dog this afraid I would be willing to leave at home and not take on walks. There are dogs like that.. happy as clams never to go anywhere. I have an older dog here that is nervy and prefers to stay home. Actually, she LOVES to ride in the car but just as soon NOT get out at a destination. She does not curl her tail under but she licks her lips, pants, glances away.. shows me in every way that she rather not be there.
Medication is an option for sure. This dog is very fearful and medication to take the edge off may be your answer and it may be life long. I would not hesitate to use medication to help her. There is a thread here "Medicating Molly." Look at it. You may need something stronger. The trick is to find the right combination so she can be happier and safer and you can enjoy her more.
If you don't want to do medication, then you need to do all you can to manage her environment and try to make her as comfortable as possible. At this age she is growing up. Her puppy play is disappearing and her adult behavior, which is largely genetic, is coming through. I wish you the best with your dog.