She tends to be aggressive with food around other dogs too. At a play date she wouldn't allow the other dog to drink water.
She also tends to be a bit bossy with other dogs more submissive than her.
What would you do for the play date and water issue? I usually grab her leash or collar and hold her back and allow the water dish to be shared.
First, I would provide her with her own water and give it to her away from the other dogs.
Sharing water with dogs in a context such as this is not one that I personally consider a ditch to die in.
I would teach her that seeing the other dogs go for the communal water is a cue to head to another area for her own water. That should be a pretty easy structure to establish. If it is not, then groundwork needs to be done away from that context.
Once she knew that she was going to get her own water in her own area, I would expect to see any hypervigilance toward other dogs drinking start to diminish. If I didn't, then I would look more closely at the dynamic in that setting to decide if it is really one that I want to expose my dog to on a regular basis.
As far as playing too strongly with softer dogs on play dates, I would probably ask those who gather for the play dates to set up a time to play with the dogs with whom my dog plays well and appropriately, and omit the softer dogs from those play sessions. Perhaps integrate some of the softer dogs back in later after my dog has had a chance to establish a rapport with some of the other stronger dogs, depending on whether or not that would really be necessary/appropriate.
Regarding growling at the vet, or any other person, I consider that clear communication on the part of the dog, indicating clearly that the dog is uncomfortable. I would approach that by implementing some desensitization/counter conditioning to create a higher level of comfort in those situations. The vet may never be a person that some dogs love to encounter, but I would build toward a level of trust and acceptance of the situation.
I have done this. Speedy had to be muzzled at the vet when he was younger. Now I can send him to the back with the techs for bloodwork and I don't have to worry in the least (unmuzzled). This happened by building trust.
Some resources you might want to check out: "Plenty in Life is Free" by Kathy Sdao (very inexpensive); "For the Love of a Dog" by Patricia McConnell (much bigger book, but well worth the time); "Reaching the Animal Mind" by Karen Pryor. None of these are actually dog training books per se, but I believe that any and all of them would help you have a better handle on what is likely happening here and give you some big-picture options on how to approach it.
Also, "Click to Calm" by Emma Parsons. This book has the absolute best protocol I've come across for dealing with resource guarding, such as what you describe with the water, etc., and would also address the issues that you describe with the play dates, and also the vet. In fact, this might be an excellent place to start, as it would touch on all of the questions that you asked.
I wish you the best with Molly.