You are half right, there are three different ground systems in play and each has their own requirements. However, "mixing" is the wrong word here. To be properly grounded, they must be connected together. Although, I do know that over in your neck of the world there are some, usually newer, houses with floating ground systems that aren't directly earthed, so in your case there may be some differences. While familiar with the concept, I have no direct experience working with such systems.
Not if you build the system properly. Just because there is a connection to an RF ground does not automagically mean RF will get into your other ground systems. As a matter of fact, is you connect them properly there is no danger of such a problem.
This is widely believed, but actually not correct either. If everything in your house is connected to a proper earth ground, then everything in the house will be at a common potential. The floating ground systems that some houses in your area have are actually based on this very concept, even if they are not at earth potential they are at the same potential. The reference (aka ground) being at the same potential is the key to lightening safety.
If you hook it up properly, there is no danger of AC mains voltage on the case of your radio gear. Why would you hook up a hot lead to a ground anyway are you trying to cause damage? Here in the states, the neutral wire is hooked up to an earth ground where the electrical circuit enters the house. This so called dangerous setup is standard procedure, and if it isn't done the person who didn't do it can be sued for violating electrical safety practices...
In most setups there is a direct DC connection between this RF ground you mentioned, and at least one other ground system, even if you didn't directly run a wire for this purpose yourself...
