I spent some time grooming, and lots of people shave down or just shorten to control shedding, but as another poster said, the fur doesn't grow back as nicely. IMO because most dogs are malnourished because of commercial food sources, but that's another post. It will not affect the dog's health in any other way, but the grown back fur tends to be coarser, doesn't lay down as nicely, and just generally not as nice looking.
If it were my dog, and you din't mind the shave, I would go to the groomer for a shave down, a #7 blade, and you'll have about labrador coat length then. In about 6-8 weeks you'll have a long haired dog again. As the coat grows back though, a little yogurt added to the meals, or some fish oil, or something with Vit E, will help the coat condition. Again, that is just what I would do.
Oh, and to brush out a Keeshound completely would take about 2 hours give or take. so you may just not be getting it all at one sitting. When you run the rake through the coat it will bring out the loose hair, keep to the same spot till you have it all out, but be careful to not irritate the skin. You can make a dog's skin bleed by over-brushing. The areas where your dog is going to have the most undercoat is on the sides and back, especially near the tuckup of the hind legs, on the chest and by the front shoulder blades.
Julie