It would be very cost prohibitive project. It would involve completely rewireing your home. i would imagine that it could run 10-30,000 depnding on the home and number of locations to be redone.
I have trouble understanding your question. But I think you are saying that you have two switches that control one light and want to know why it is not working. More than likely is that you have two way switches instead of 3 way switches installed. 3 three switches cost a little more and are a little more difficult to install but if you recently replaced these switches you may have replaced them with the wrong type.
You will actually need 2 3-way switches to control something from two locations. The first switch will receive the power, while the other will go directly to the load being controlled. The switch getting power will have the hot lead connected to the common terminal. The other switch will have its common terminal connected to the hot lead going to the load. Two "travellers" will be connected between the other terminals on the switches. They will be connected switch-to-switch. And don't forget that your neutral coming from the power feed will be connected to the load. Also don't forget to connect ground wires.
Yes. the two switches are called '3-way' switches. You will have to turn the fan/light on with one of the switches and leave BOTH switches in that position. The remote control unit will have to be wired in between the switch that is supplying power to the devices and the devices. This is the simplest way. You could also disable one of the switches and replace the remaining switch with the RC unit. If you are not sure how to deal with 3-way switches, learn about them first, or call a good electrician to do all of the work. Remeber that the black wire is hot, but only if the house was wired properly in the first place.
No. A double pole switch would be a switch that switches 2 different loads simultaneously. 3-way switches (and 4-way switches if more than 2 are used) are what are used for controlling a light from multiple locations. While a typical single pole switch simply opens or closes a single contact, a 3-way switch provides a path from a contact point to, let's say, point A or point B. Points A and B from the first 3-way switch are connected to points A and B of the second 3-way switch and then the common contact point of the second switch is connected to your light. When more than 2 switches are desired, you can use any number of 4-way switches between the 3-way switches, but the 3-way switches MUST be on the ends of the circuit.