Can a car's keyless remote entry signal be sniffed and reproduced?

Yes, but more expensive units will change the frequency every time you use it.

With any thing electronic like that yes it can. However its not easy. If you have a late model car the codes are changed every time you shut the car off. The system also will unlock but not start if the rest of the information the key has does not match the ECU.

Yes. You can buy the equipment off the internet. It's very expensive so unless you own a really high end car, I would count on the more traditional methods of theft.

Alot of the newer (sometimes pricier) systems have what is called "rolling codes". this means that the reciever in the system and the transmitter in your remote are syncronized to both change the code everytime the system is armed/disarmed. they have to be syncronized together so that they will both "roll" to the same code when they change.

YES! IF IT USES A RADIO SIGNAL,, IT CAN BE REPRODUCED

I do't think you can with the Mercedes system.