1. not sure about the legal situation, but it s unethical for sure. but I can remember one gentlemen went to courts regarding the same issue against a car seller ( it was in the newspaper)
2. in some vehicles, yes - you might be able to do it when the car is with multiple memory places, provided the fellow has not change them all.
eg: freelander 2, but for the models you are behind - I dont think so
3. all the agents do issue inspection certificates, in fact nowadays most of the buyers do that especially when buying hybrids, so I recommend the relevant agent for that matter
4. price - varies, depending on the agent, model etc, so you need to give 2 telephone calls to the 2 agents