The short answer is: you can't. Police dogs are very highly trained by experts. Police departments don't buy police dogs from private parties, only from special training schools.
Yes, women can work on the K-9 unit. I have never heard of a department that will not allow that.
Many people are unaware of this, but the K-9 unit requires any candidates to pass a very difficult physical test. The reason being that as a K-9 officer, you will be in control of a large German shep and there may be time where you may have to actually lift the dog eg. if need to get the dog over a fence but still maintain control without unleashing.
As long as the physical can be passed, any candidate is eligible.
yes
It is unlikely. While not all dogs used by law enforcement are not necessarily large, strong breeds, the ones that are specialized, (drug dogs - bomb dogs - cadaver dogs - search dogs - etc) are usually picked due to the fact that a particular breed is known to have or bred to possess exceedingly good powers of scent identification. I am not aware that Corgi's fall into this category.
retired police dogs will be protective of their owner, but it really depends on how old they are