It is the norm to leave your dog in your car when you aren't actively working them on the field. I know at my club when we have new people coming to check us out they always have their dog out the entire time with them watching everything and before long that puppy is bored/tired with standing around. It's best to leave them in the car and have them fresh and excited when you bring them out. At the schutzhund field a worn out dog isn't a good thing, only after you get home from training is it a good thing.
And yes, helpers are the key to your dogs progress. It will take a while of watching different helpers work dogs before you get a feel for what you like and don't like for your dog. I have worked Cher with 4 helpers so far and they are all VERY different. One I won't ever bother to work her on again, one I will work her on only occassionally but I don't expect much advancement from, and two are very good and the ones I work her on regularly. It took some time with the club before I was able to distinguish what I personally was looking for in a helper though, and it can be a little different for each handler and dog.