Fetch is a learned behavior. Very few dogs, even retrievers, get the bringing it back part without some training. The dogs are likely viewing your hands as toys, part of the fun. I'd treat like training a puppy not to nip...when it happens, no matter how soft the nip is, yelp like they ripped our hand off, turn your back and ignore the dog as much as possible. Nothing most dogs hate more than being ignored. Don't respond until they settle down again.

As for the one who grabbed while you took the leash off, lots of shelter dogs have serious issues with their collars being touched, especially if done from over top of their heads. Many of my dogs are shelter dogs and I'm sure that some of them were dragged by their collars. Founder used to do the exact same thing you described, every single time I touched his collar. Hoodoo would go limp and refuse to move. Touch that area slowly, gently, and as calmly as possible and frequently. Make it as pleasurable as possible so the association will change to one that's positive. When putting a leash on or off, don't come over the top of the dog. That's a highly dominant position and will make an already nervous dog freak out. Get down to their level, reach under their head, and move slowly while talking in a happy voice.

I've had no success at stopping dogs from eating grass! Some of mine graze like cows, some hardly ever eat greens! As long as they aren't eating foxtails(No idea what they would be called or if they exist in England), I just let them graze.