"What is balanced dog training?"
Balanced dog training is the practice of using both negative and positive reinforcement to communicate with dogs. Dogs naturally learn and communicate with one another off of different types of pressure. Dogs also crave and thrive structure. The dog learns doing the right thing gets rewarded, and doing the wrong thing gets corrected. There's a balanced of both positive and negative reinforcement. 
Examples of positive reinforcement-Good dog, treats, pets, toys, pressure release. Examples of negative reinforcement- NO, leash pressure/pop, spacial pressure.

"Are e-collars cruel? Will it hurt and scare my dog?"
No, e-collars are not cruel and actually help dogs live much more fulfilling lives by providing off leash safety/reliability. With the proper conditioning, dogs end up having a very positive association with the stim because they get positive reinforcement, and always end up doing something fun with the collar on. It can be used negatively which is why there's a lot of debate, unfortunately anyone can buy one. You can build a house with a hammer, you can break a house with a hammer.

"Are prong collars and slip leads abusive?"
No, just like the e-collar it all depends on how they are used. Both provide even pressure around the dog's neck when they pull, dogs learn that yielding to pressure gives them big rewards. In the end it saves their neck and allows you to have full control of the dog with proper training.

"My dog is 3, is he too old to train?"
No dog is too old to train, just like us we are always capable of learning new things, I've had 10 YO dogs come in for work and completely change their bad habits they're had their whole life.

"I bought a no-pull harness, will it help?"
No, the harness is deflecting an issue not addressing it. Harnesses should only be used for working dogs, harnesses are meant to allow a dog to pull with all their body weight comfortably, when restricted it just makes the dog want to pull harder, this is known an oppositional reflex. Most pet harnesses also restrict movement which leads to joint issues long term.